Previously, SampleIntegrationParentApplicationTests assumed that when
an output file existed on disk its contents would have been written
in their entirety. This assumption does not hold true and causes the
test to fail intermittently as it incorrectly determines that the test
has produced no output.

This commit updates the test to wait for up to 30 seconds for the
output files to appear on disk and for the expected content to be
found in one of those files. If the files exist but do not
contain the expected content the test will keep trying until it does
or until too much time as elapsed.

Fixes gh-2380