kunit: tool: simplify creating LinuxSourceTreeOperations

Drop get_source_tree_ops() and just call what used to be
get_source_tree_ops_from_qemu_config() in both cases.

Also rename the functions to have shorter names and add a "_" prefix to
note they're not meant to be used outside this function.

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