From the compiler distribution directory, going to

  testsuite/

and running

  make one DIR=tests/tools-toplevel

I observe various output (actually in the run I made I can see this failure line as well). The test that fails in my case is find-artifacts/, so there is a log directory

  tests/tool-debugger/find-artifacts/_ocamltest/

In particular there is a file

  cat tests/tool-debugger/find-artifacts/_ocamltest/tests/tool-debugger/find-artifacts/debuggee/ocamlc.byte/debuggee.byte.output

which ends with

Debugger [version 4.10.0+dev0-2019-04-23] environment error:
 The files /usr/local/lib/ocaml/stdlib.cmi
 and /home/gasche/Prog/ocaml/github-trunk/testsuite/tests/tool-debugger/find-artifacts/_ocamltest/tests/tool-debugger/find-artifacts/debuggee/ocamlc.byte/out/blah.cmi
 make inconsistent assumptions over interface Stdlib

This seems to be a problem where the testsuite environment is not sufficiently isolated from the outside world, and another OCaml install somewhere breaks things.