Tests: Rename self.diag to self.tap and use self.t.diag where applicable

Having self.diag and self.t.diag was confusing as one referred to TAP
output (self.diag) while the other referred to "task diagnostics"
self.t.diag.

self.diag is now gone and was replaced by self.tap with the same
behavior.

test/template.t

@@ -60,7 +60,7 @@ class TestBugNumber(TestCase):
         self.assertRegexpMatches(out, expected)
 
         # TAP diagnostics on the bas
-        self.diag("Yay TAP diagnostics")
+        self.tap("Yay TAP diagnostics")
 
     def test_faketime(self):
         """Running tests using libfaketime"""