tests: use an all too familiar executable in test-run-tests.t (issue6661)

true(1) sometimes lives in /usr/bin/, and so this test fails on such systems.
We also can't use which(1), because it's apparently not POSIX and Debian
complains about it [1]. We also cannot really create a script and chmod +x it,
because this is a symlink case, execbit case is slightly below. So let's use
something that we know is executable, but not hg itself.

tests/test-run-tests.t

@@ -21,9 +21,9 b' Define a helper to avoid the install ste'
 error paths
 
 #if symlink
-  $ ln -s "/"bin"/"true hg
+  $ ln -s $TESTDIR/run-tests.py hg
   $ "$PYTHON" $TESTDIR/run-tests.py --with-hg=./hg
-  ^warning: --with-hg should specify an hg script, not: (true|coreutils)$ (re)
+  warning: --with-hg should specify an hg script, not: run-tests.py
   running 0 tests using 0 parallel processes 
   
   # Ran 0 tests, 0 skipped, 0 failed.