tools: Change coverage tool to work with '.coverage' file.

- Was generating coverage by running a corresponding test file
  for the current file.  Wasn't obvious where test file was.
- This shows coverage if a .coverage file exists which is newer
  than the current file.
- The .coverage file needs to be created independantly. And then
  the reports can be refreshed.

tests/golden-files/results/python_coverage-hi3_py

@@ -1,5 +0,0 @@
-  
-  
-> def hi():
->     print("hi")
-

tests/tools_test.py

@@ -106,9 +106,6 @@ class ToolsTestCase(unittest.TestCase):
     def test_mypy(self):
         self._test_tool(tools.mypy, self.HI_OK)
 
-    def test_python_coverage(self):
-        self._test_tool(tools.python_coverage, self.HI_NORMAL)
-
     def test_pycodestyle(self):
         self._test_tool(tools.pycodestyle, self.HI_OK)