tests: Fix JUNIT_OUTPUT=y to write to files instead of stderr

tests: Fix JUNIT_OUTPUT=y to write to files instead of stderr

CB:57144 broke JUNIT_OUTPUT=y, and unit-tests were writing to stderr
instead of selected XML files, because test name used for XML file
creation contains test path. Build system did not create necessary
directiories, which CMocka required to create output files.
This commit fixes writing to XML files with JUNIT_OUTPUT=y, but had to
sacrifice path in test name, as it was causing a lot of problems
(because CMocka does not know, how to write multiple test groups to one
XML file, so it uses test group name [here __TEST_NAME__(test_group)] as
part of output filename).
Example:
 Test: tests/lib/rtc-test
 Output file:
 `build/tests/junit-tests_lib_rtc-test(tests).xml

Change-Id: I09891aca923bf1271cafeaa09f89b6539022709c
Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58163
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>