run-tests: obtain code coverage via Python API

Before, we were invoking the "coverage" program provided by the
"coverage" module. This patch changes the code to go through the
Python API. This makes the next patch a little bit easier to reason
about.

A side effect of this patch is that writing code coverage reports
will be slightly faster, as we won't have to redundantly load
coverage data.

tests/run-tests.py

@@ -1985,27 +1985,27 b' class TestRunner(object):'
 
     def _outputcoverage(self):
         """Produce code coverage output."""
-        vlog('# Producing coverage report')
-        os.chdir(self._pythondir)
+        from coverage import coverage
 
-        def covrun(*args):
-            cmd = 'coverage %s' % ' '.join(args)
-            vlog('# Running: %s' % cmd)
-            os.system(cmd)
+        vlog('# Producing coverage report')
+        # chdir is the easiest way to get short, relative paths in the
+        # output.
+        os.chdir(self._pythondir)
+        covdir = os.path.join(self._installdir, '..')
+        cov = coverage(data_file=os.path.join(covdir, '.coverage'))
+        cov.load()
 
-        covrun('-c')
-        omit = ','.join(os.path.join(x, '*') for x in
-                        [self._bindir, self._testdir])
-        covrun('-i', '-r', '"--omit=%s"' % omit) # report
+        omit = [os.path.join(x, '*') for x in [self._bindir, self._testdir]]
+        cov.report(ignore_errors=True, omit=omit)
+
         if self.options.htmlcov:
             htmldir = os.path.join(self._testdir, 'htmlcov')
-            covrun('-i', '-b', '"--directory=%s"' % htmldir,
-                   '"--omit=%s"' % omit)
+            cov.html_report(directory=htmldir, omit=omit)
         if self.options.annotate:
             adir = os.path.join(self._testdir, 'annotated')
             if not os.path.isdir(adir):
                 os.mkdir(adir)
-            covrun('-i', '-a', '"--directory=%s"' % adir, '"--omit=%s"' % omit)
+            cov.annotate(directory=adir, omit=omit)
 
     def _findprogram(self, program):
         """Search PATH for a executable program"""