tests: fix parsing w/ path containing "-pa" or "-pS"

The OutputTagExtract test case for tiff_test runs exiv2 two times,
with "-pa" and "-pS", and parses their outputs. To know which output to
parse, it checks for "-pa" and "-pS" in the string of the command being
run; considering that the command string contains the full path to the
test data, which is a subdirectory of the sources, this means that a
wrong parser will be used in case the full source path contains any of
"-pa" or "-pS" (e.g. "/build/some-path/exiv2/...").

Cheap fix for this: since "-pa"/"-pS" are options in the command string,
check for them using spaces around.

tests/tiff_test/test_tag_compare.py

@@ -98,9 +98,9 @@ class OutputTagExtract(metaclass=system_tests.CaseMeta):
     def compare_stdout(self, i, command, got_stdout, expected_stdout):
         super().compare_stdout(i, command, got_stdout, expected_stdout)
 
-        if '-pa' in command:
+        if ' -pa ' in command:
             self.pa_data = self.parse_pa(got_stdout.splitlines())
-        if '-pS' in command:
+        if ' -pS ' in command:
             self.pS_data = self.parse_pS(got_stdout.splitlines())
 
         if i == 1: