tests: allow the true command to be provided by coreutils

The `true` command is sometimes provided as a symbolic link to the `coreutils`
single binary. This is the case on NixOS, on which the test was failing because
the symbolic link fully resolves to the latter name, equally valid but not
previously accepted by the test's golden output.

Differential Revision: https://phab.mercurial-scm.org/D11740

tests/test-run-tests.t

@@ -23,7 +23,7 b' error paths'
 #if symlink
   $ ln -s `which true` hg
   $ "$PYTHON" $TESTDIR/run-tests.py --with-hg=./hg
-  warning: --with-hg should specify an hg script, not: true
+  ^warning: --with-hg should specify an hg script, not: (true|coreutils)$ (re)
   running 0 tests using 0 parallel processes 
   
   # Ran 0 tests, 0 skipped, 0 failed.