selftests: Move KTAP bash helpers to selftests common folder

Move bash helpers for outputting in KTAP format to the common selftests
folder. This allows kselftests other than the dt one to source the file
and make use of the helper functions.
Define pass, fail and skip codes in the same file too.

Signed-off-by: Laura Nao <laura.nao@collabora.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>