kunit: test: Make filter strings in executor_test writable

KUnit's attribute filtering feature needs the filter strings passed in
to be writable, as it modifies them in-place during parsing. This works
for the filters passed on the kernel command line, but the string
literals used in the executor tests are at least theoretically read-only
(though they work on x86_64 for some reason). s390 wasn't fooled, and
crashed when these tests were run.

Use a 'char[]' instead, (and make an explicit variable for the current
filter in parse_filter_attr_test), which will store the string in a
writable segment.

Fixes: 76066f93

 ("kunit: add tests for filtering attributes")
Closes: https://lore.kernel.org/linux-kselftest/55950256-c00a-4d21-a2c0-cf9f0e5b8a9a@roeck-us.net/
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>