run-test.sh: treat empty gzipped output files as text files
run-test.sh: treat empty gzipped output files as text files

I saw a test run where pstack_watcher-test timed out and gzip didn't produce
any output. Thus, we ended up with an empty pstack_watcher-test.txt.gz file
which is corrupt by definition; there should at least be a gzip header.

If this happens, let's treat the empty file as an empty text file so that
subsequent output file post-processing (which uses commands like zcat/zgrep)
doesn't fail.

Importantly, the pstack_watcher-test timeout does NOT benefit from this fix.
That's because pstack_watcher-test was being traced by gdb, so it couldn't
deliver itself a SIGABRT to timeout after 900 seconds. Instead, ctest killed
it after 930 seconds by sending a SIGKILL to run-test.sh itself, thus
preventing this cleanup code from running.

Change-Id: I0fbe13982412f9113b27afbc40635f8945c45f09
Reviewed-on: http://gerrit.cloudera.org:8080/10062
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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