Ensure 'subunit2pyunit' is run in venv from run_tests.sh

The run_tests.sh script invokes

  bash -c "${wrapper} $TESTRTESTS | subunit2pyunit"

where '${wrapper}' expands to 'tools/with_venv.sh'. Due to shell
precedence rules, this only applies to '$TESTRTESTS' not the
'subunit2pyunit' part of the command. The 'subunit2pyunit' binary
though may only be available from the .venv

Change-Id: I5972c8f77b3f50a3713881e6ca1acbaeabbd47d4
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>