Fail when no test case selected

When the list based filtering logic was used
and no test was selected at the end.
testr was invoked to run all test.

For example:
ostestr --regex a --black-regex a

Change-Id: I7b580c3954f71dc5fe347b82d6d14004bfdb8f22

os_testr/ostestr.py

@@ -256,6 +256,10 @@ def _call_testr_with_list(opts, test_list, others):
         print("\n".join(test_list))
         return 0
 
+    if not test_list:
+        print("No testcase selected to run")
+        return 8
+
     ec = call_testr(None, opts.subunit, opts.pretty, opts.list,
                     opts.slowest, opts.parallel, opts.concurrency,
                     opts.until_failure, opts.color, test_list, others)

os_testr/tests/test_return_codes.py

@@ -102,3 +102,6 @@ class TestReturnCodes(base.TestCase):
 
     def test_list(self):
         self.assertRunExit('ostestr --list', 0)
+
+    def test_no_test(self):
+        self.assertRunExit('ostestr --regex a --black-regex a', 8)