selftests: ktap_helpers: Make it POSIX-compliant

There are a couple uses of bash specific syntax in the script. Change
them to the equivalent POSIX syntax. This doesn't change functionality
and allows non-bash test scripts to make use of these helpers.

Reported-by: Mike Looijmans <mike.looijmans@topic.nl>
Closes: https://lore.kernel.org/all/efae4037-c22a-40be-8ba9-7c1c12ece042@topic.nl/


Fixes: 2dd0b5a8 ("selftests: ktap_helpers: Add a helper to finish the test")
Fixes: 14571ab1 ("kselftest: Add new test for detecting unprobed Devicetree devices")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>