kunit: tool: print out stderr from make (like build warnings)

kunit: tool: print out stderr from make (like build warnings)

Currently the tool redirects make stdout + stderr, and only shows them
if the make command fails.
This means build warnings aren't shown to the user.

This change prints the contents of stderr even if make succeeds, under
the assumption these are only build warnings or other messages the user
likely wants to see.

We drop stdout from the raised exception since we can no longer easily
collate stdout and stderr and just showing the stderr seems fine.

Example with a warning:

[14:56:35] Building KUnit Kernel ...
../lib/kunit/kunit-test.c: In function ‘kunit_test_successful_try’:
../lib/kunit/kunit-test.c:19:6: warning: unused variable ‘unused’ [-Wunused-variable]
   19 |  int unused;
      |      ^~~~~~

[14:56:40] Starting KUnit Kernel ...

Note the stderr has a trailing \n, and since we use print, we add
another, but it helps separate make and kunit.py output.

Example with a build error:

[15:02:45] Building KUnit Kernel ...
ERROR:root:../lib/kunit/kunit-test.c: In function ‘kunit_test_successful_try’:
../lib/kunit/kunit-test.c:19:2: error: unknown type name ‘invalid_type’
   19 |  invalid_type *test = data;
      |  ^~~~~~~~~~~~
...

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>