tests: hint how to run slow tests when rejecting

tests/hghave.py

@@ -605,7 +605,7 b' def has_pure():'
         os.environ.get("HGTEST_RUN_TESTS_PURE") == "--pure",
     ])
 
-@check("slow", "allow slow tests")
+@check("slow", "allow slow tests (use --allow-slow-tests)")
 def has_slow():
     return os.environ.get('HGTEST_SLOW') == 'slow'
 

tests/test-run-tests.t

@@ -555,7 +555,7 b' timeouts'
   > cat test-timeout.t >> test-slow-timeout.t
   $ rt --timeout=1 --slowtimeout=3 test-timeout.t test-slow-timeout.t
   st
-  Skipped test-slow-timeout.t: missing feature: allow slow tests
+  Skipped test-slow-timeout.t: missing feature: allow slow tests (use --allow-slow-tests)
   Failed test-timeout.t: timed out
   # Ran 1 tests, 1 skipped, 0 warned, 1 failed.
   python hash seed: * (glob)
@@ -852,7 +852,7 b' test support for --allow-slow-tests'
   > EOF
   $ rt test-very-slow-test.t
   s
-  Skipped test-very-slow-test.t: missing feature: allow slow tests
+  Skipped test-very-slow-test.t: missing feature: allow slow tests (use --allow-slow-tests)
   # Ran 0 tests, 1 skipped, 0 warned, 0 failed.
   $ rt $HGTEST_RUN_TESTS_PURE --allow-slow-tests test-very-slow-test.t
   .