hghave: add test for whether fsmonitor is enabled

This uses the HGFSMONITOR_TESTS environment variable that
fsmonitor-run-tests.py adds.

tests/hghave.py

@@ -643,3 +643,7 b' def has_virtualenv():'
         return True
     except ImportError:
         return False
+
+@check("fsmonitor", "running tests with fsmonitor")
+def has_fsmonitor():
+    return 'HGFSMONITOR_TESTS' in os.environ