[ISSUE] Several tests fail when stdin is not a tty

[qsuscs <qsuscs@users.noreply.github.com>]

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New issue by qsuscs on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/200

Description:
Several tests fail when being executed by Gentoo’s build infrastructure, which does not hook stdin to a tty.  I noticed this because of `t/3000-magrep.t`, since [line 234 of `magrep.c`](https://github.com/leahneukirchen/mblaze/blob/v1.1/magrep.c#L234) checks if fd 0 is a tty.

This can easily be reproduced without Gentoo:
```

qsx@coruscant ~/src/mblaze (git)-[tags/v1.1] 
% make check </dev/null 
PATH=$(pwd):$PATH prove -v
t/1000-mmime.t .......... 
1..8
ok - mime -r runs
ok - no overlong lines
ok - no QP when unecessary
ok - no further mime necessary
ok - include works
ok - include sets filename
ok - include works, overriding filename
ok - include works, overriding filename
ok
t/1500-maddr.t .......... 
1..11
ok - from one
ok - from address
ok - from one
ok - from two
ok - from addr only
ok - from name only
ok - specific header
ok - long addr
ok - decode iso8859
ok - decode long iso8859
ok - decode utf8
ok
t/1501-maddr-regress.t .. 
1..33
ok - parse foo@example.org
ok - parse <foo@example.org>
ok - parse bar <foo@example.org>
ok - parse bar quux <foo@example.org>
ok - parse bar  quux <foo@example.org>
ok - parse <foo@example.org> (bar)
ok - parse "Real Name" <foo@example.org>
ok - parse "Dr. Real Name" <foo@example.org>
ok - parse "Real Name" (show this) <foo@example.org>
ok - parse "Real Name" <foo@example.org> (ignore this)
ok - parse (nested (comments mail@here ) heh) "yep (yap)" <foo@example.org>
ok - parse ignore-this: <foo@example.org>
ok - parse ignore-this : <foo@example.org>
ok - parse "foo"@example.org
ok - parse "Barqux" "foo"@example.org
ok - parse Space Man <"space man"@example.org>
ok - parse Space Man <"space  man"@example.org>
ok - parse <spaceman@example.org  >
ok - parse < spaceman@example.org  >
ok - parse <spaceman@(wtf)example.org  >
ok - parse space\man@example.org
ok - parse what is <"this<evil(h\"eh)sh>it"@example.org>
ok - parse foo@example.org <foo@example.org>
ok - parse foo@[::1] (ipv6)
ok - parse <Foo Bar <foobar@qux.com>
ok - parse "abc@def"@ghi
ok - parse "foo@" <bar.com foo@bar.com>
ok - parse test."test"@example.org
ok - parse <test."test"@example.org>
ok - parse test"test"@example.org
ok - parse <test"test"@example.org>
ok - parse foo<bar@example.org>
ok - parse xxxxxxxxx a"test"@example.org
ok
t/1700-mshow.t .......... 
1..2
ok - nested mail has 5 attachments
ok - nested mail has text/html attachment
ok
t/2000-mpick.t .......... 
1..29
ok - flag trashed
ok - flag not trashed
ok - flag seen
ok - flag not seen
ok - flag seen and trashed
ok - flag seen and not trashed
ok - flag replied
ok - flag forwarded
ok - search subject
ok - search addr
ok - search name
ok - search spam
ok - any header
ok - addr decode addr
ok - addr decode disp
ok - shebang
ok - ternary
ok - pipe command
ok - create file
ok - overwrite file
ok - append file
ok - unknown ident
ok - let expression
ok - let expression double free
ok - let expression nested
ok - let scoping
ok - multi redir
ok - multi redir prefixes
ok - environment variable
ok
t/3000-magrep.t ......... 
1..10
not ok - subject
Dubious, test returned 1 (wstat 256, 0x100)
Failed 10/10 subtests 
t/4000-msed.t ........... 
1..6
ok - append new
ok - append existing
ok - append multiple
ok - change
ok - delete
ok - substitute
ok
t/5000-mscan.t .......... 
1..3
ok - ISO date
ok - from name
ok - multiple mmsg
ok
t/6000-msort.t .......... 
1..5
--- out1	2021-01-27 17:59:14.584305434 +0100
+++ out2	2021-01-27 17:59:14.590305417 +0100
@@ -0,0 +1,4 @@
+inbox/cur/1:2,
+inbox/cur/2:2,
+inbox/cur/3:2,
+inbox/cur/4:2,
not ok - filename
Dubious, test returned 1 (wstat 256, 0x100)
Failed 5/5 subtests 
t/7000-mseq.t ........... 
1..11
ok - set current
ok - set next
ok - set prev
ok - last
ok - whole thread
ok - subthread
ok - parent
ok - range
ok - multiple mmsg
ok - whole thread at the end
ok - subthread at the end
ok
t/8000-mflag.t .......... 
1..16
ok - mark seen
not ok - fix seq
Dubious, test returned 1 (wstat 256, 0x100)
Failed 15/16 subtests 
t/9000-minc.t ........... 
1..1
ok - minc
ok

Test Summary Report
-------------------
t/3000-magrep.t       (Wstat: 256 Tests: 1 Failed: 1)
  Failed test:  1
  Non-zero exit status: 1
  Parse errors: Bad plan.  You planned 10 tests but ran 1.
t/6000-msort.t        (Wstat: 256 Tests: 1 Failed: 1)
  Failed test:  1
  Non-zero exit status: 1
  Parse errors: Bad plan.  You planned 5 tests but ran 1.
t/8000-mflag.t        (Wstat: 256 Tests: 2 Failed: 1)
  Failed test:  2
  Non-zero exit status: 1
  Parse errors: Bad plan.  You planned 16 tests but ran 2.
Files=12, Tests=108,  1 wallclock secs ( 0.14 usr  0.04 sys +  0.56 cusr  1.04 csys =  1.78 CPU)
Result: FAIL
make: *** [GNUmakefile:53: check] Error 1
```