tests: delete parallel coverage files after combining

tests/run-tests.py

@@ -331,6 +331,10 b' def outputcoverage(options):'
     omit = ','.join(omit)
 
     covrun('-c') # combine from parallel processes
+    for fn in os.listdir(TESTDIR):
+        if fn.startswith('.coverage.'):
+            os.unlink(os.path.join(TESTDIR, fn))
+
     covrun('-i', '-r', '"--omit=%s"' % omit) # report
     if options.annotate:
         adir = os.path.join(TESTDIR, 'annotated')