Coverage: Exclude migrations and management commands.

These are not particularly interesting to measure test coverage for,
since migrations are not run as the part of the test suite by
construction, and management commands aren't being tested by this test
suite.

tools/test-backend

@@ -142,7 +142,9 @@ if __name__ == "__main__":
 
     if options.coverage:
         import coverage
-        cov = coverage.Coverage(omit="*/zulip-venv-cache/*")
+        cov = coverage.Coverage(omit=["*/zulip-venv-cache/*",
+                                      "*/migrations/*",
+                                      "*/management/commands/*"])
         cov.start()
     if options.profile:
         import cProfile