tests: let run-tests.py default to use 'sh' in $PATH instead of '/bin/sh'

This makes it easier to run tests on systems that have a usable sh in the
search path but not in the standard location.

tests/run-tests.py

@@ -98,7 +98,7 defaults = {
     'jobs': ('HGTEST_JOBS', 1),
     'timeout': ('HGTEST_TIMEOUT', 180),
     'port': ('HGTEST_PORT', 20059),
-    'shell': ('HGTEST_SHELL', '/bin/sh'),
+    'shell': ('HGTEST_SHELL', 'sh'),
 }
 
 def parselistfiles(files, listtype, warn=True):