testing: Write the .coverage file.

We need to call save explicitly to make coverage write the
.coverage file.

tools/test-backend

@@ -328,6 +328,8 @@ if __name__ == "__main__":
         cov.stop()
         cov.save()
         cov.combine()
+        cov.data_suffix = False  # Disable suffix so that filename is .coverage
+        cov.save()
         if options.verbose_coverage:
             print("Printing coverage data")
             cov.report(show_missing=False)