tests: Ensure test files are newline-terminated

tests: Ensure test files are newline-terminated

Currently we only append a newline to 'actual' if 'expected'
(as loaded from file) already ends in a newline, but that
results in inconsistent behavior.

For example, some of the test files used by virhostcputest are
newline-terminated and some aren't. If we were to remove
existing newlines from those files or add them where they
aren't present, the test would still pass, and even using
VIR_TEST_REGENERATE_OUTPUT=1 wouldn't change them back.

Make things consistent by ensuring that 'actual' is always
newline-terminated. The only exception is when 'actual' is
completely empty: in that case, we want the file to be actually
empty, not contain a single empty line. query-jobs-empty.result
in qemumonitorjsondata/ is an example of this being used.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>