Ensure filehandles are closed, processes are finished

[llimeht]

The test suite generates a lot of ResourceWarning errors from filehandles that are not closed or subprocess.Popen calls that are not correctly finalised.

Example:

unittests/FunctionalTests.py::test
  /tmp/autopkgtest.AfpzrA/autopkgtest_tmp/unittests/FunctionalTests.py:96: ResourceWarning: unclosed file <_io.TextIOWrapper name='/tmp/autopkgtest.AfpzrA/autopkgtest_tmp/unittests/Packages/benchmarks/babel.html' mode='r' encoding='ANSI_X3.4-1968'>
    bench = open(benchfile).readlines()

unittests/FunctionalTests.py::test
  /usr/lib/python3.7/subprocess.py:883: ResourceWarning: subprocess 392 is still running
    ResourceWarning, source=self)

This PR uses context managers for the file handles and calls wait() to reap the subprocess correctly.