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/basetest/testing.py

@@ -7,11 +7,11 @@ from .taskd import Taskd
 
 
 class BaseTestCase(unittest.TestCase):
-    def diag(self, out):
-        sys.stderr.write("--- diag start ---\n")
+    def tap(self, out):
+        sys.stderr.write("--- tap output start ---\n")
         for line in out.splitlines():
             sys.stderr.write(line + '\n')
-        sys.stderr.write("---  diag end  ---\n")
+        sys.stderr.write("---  tap output end  ---\n")
 
 
 @unittest.skipIf(TASKW_SKIP, "TASKW_SKIP set, skipping task tests.")