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/version.t

@@ -106,9 +106,7 @@ class TestVersion(TestCase):
         """Task binary matches the current git commit"""
         expected = "Commit: {0}".format(self.slurp_git())
 
-        args = ("diag",)
-
-        code, out, err = self.t(args)
+        code, out, err = self.t.diag()
         self.assertIn(expected, out)