Ensure execrunner test goroutine exits before test
Ensure execrunner test goroutine exits before test

Also pass in defaultProcessIO containing non-nil stdin to every test
because otherwise reading blocks (execrunner doesn't close stdio pipes
if the processIO.stdios are nil for some reason)

[#153474851]

Signed-off-by: Danail Branekov <43f82d874965b9fe2876a1a864fffde1e64d6143@gmail.com>

cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian