selftests: lib: Skip tests on missing test modules

With older kernels, printf.sh and bitmap.sh fail because they can't find
the respective test modules they are looking for.

Use modprobe dry run to check for missing test_XXX module. Error out with
the same error code as prime_numbers.sh.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>