tests: Fix parallel runs of TLS test suites

tests: Fix parallel runs of TLS test suites

I noticed this yesterday and fixed it in a different way, but ended up
with one more problem.  It was probably the way I fixed it combined
with one more filename changed.

Anyway, why I'm saying this is that one more filename should be renamed
in order to avoid a race (which I was unable to reproduce, though).

I checked this is the last file those two tests have in common by going
through the code and the re-checked by this "script":

strace -o session.trace -e open ./virnettlssessiontest
strace -o context.trace -e open ./virnettlscontexttest
sort \
 <(sed -n '/^open/s/open("\([^"]*\)",.*$/\1/p' context.trace | sort -u)\
 <(sed -n '/^open/s/open("\([^"]*\)",.*$/\1/p' session.trace | sort -u)\
 | uniq -d| grep '.pem$'

So it should be enough to make these tests independent of each other.

Signed-off-by: Eric Blake <eblake@redhat.com>