tests: unset variable HG if it is set

When hg tries to call itself it can call a different hg executable when this
variable is set. Some tests fail when the called hg version is different.

tests/run-tests.py

@@ -1211,6 +1211,10 b' def main():'
             # can't remove on solaris
             os.environ[k] = ''
             del os.environ[k]
+    if 'HG' in os.environ:
+        # can't remove on solaris
+        os.environ['HG'] = ''
+        del os.environ['HG']
 
     global TESTDIR, HGTMP, INST, BINDIR, PYTHONDIR, COVERAGE_FILE
     TESTDIR = os.environ["TESTDIR"] = os.getcwd()