tools/testing/kunit/kunit.py: remove redundant double check

The build_tests function contained double checking for not success
result. It is fixed in the current patch. Additional small
simplifications of code like using ternary if were applied (avoid using
the same operation by calculation times differ in two places).

Signed-off-by: Alexander Pantyukhin <apantykhin@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>