tests: explicitly close subprocess files

Resolve the following error:
/vpp/test/framework.py:657: ResourceWarning: unclosed file <_io.BufferedReader name=6>
  del cls.vpp
Object allocated at (most recent call last):
  File "/usr/lib/python3.8/subprocess.py", lineno 844
    self.stdout = io.open(c2pread, 'rb', bufsize)

Type: test

Change-Id: Ia2974da594f0582dcff1f4bdf40d25475769c46c
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>