Return to the original directory after exec'ing a file

If the file that is exec'd chdirs to a directory that doesn't exist
at the end of the execution, then we will fail to connect to the
SQLite database (due to a failing `os.getcwd` command).

We can easily fix this if we ensure we are in a directory that
exists after executing the foreign code. Returning to the original
directory seems to be a sensible choice.

Author: hemberger

coverage/execfile.py

@@ -196,7 +196,11 @@ def run(self):
         # a non-existent directory.
         cwd = os.getcwd()
         try:
+            # Return to the original directory in case the test code exits in
+            # a non-existent directory.
+            cwd = os.getcwd()
             exec(code, main_mod.__dict__)
+            os.chdir(cwd)
         except SystemExit:                          # pylint: disable=try-except-raise
             # The user called sys.exit().  Just pass it along to the upper
             # layers, where it will be handled.