coverage: Move coverage config to tools/coveragerc.

tools/coveragerc

@@ -0,0 +1,11 @@
+[report]
+# Regexes for lines to exclude from consideration
+exclude_lines =
+    # Re-enable the standard coverage pragma
+    pragma: no cover
+
+[run]
+omit =
+    */zulip-venv-cache/*
+    */migrations/*
+    */management/commands/*

tools/test-backend

@@ -230,9 +230,7 @@ if __name__ == "__main__":
 
     if options.coverage:
         import coverage
-        cov = coverage.Coverage(omit=["*/zulip-venv-cache/*",
-                                      "*/migrations/*",
-                                      "*/management/commands/*"])
+        cov = coverage.Coverage(config_file="tools/coveragerc")
         cov.start()
     if options.profile:
         import cProfile