Convert stackinit unit tests to KUnit, for better integration
into the kernel self test framework. Includes a rename of
test_stackinit.c to stackinit_kunit.c, and CONFIG_TEST_STACKINIT to
CONFIG_STACKINIT_KUNIT_TEST.

Adjust expected test results based on which stack initialization method
was chosen:

 $ CMD="./tools/testing/kunit/kunit.py run stackinit --raw_output \
        --arch=x86_64 --kconfig_add"

 $ $CMD | grep stackinit:
 # stackinit: pass:36 fail:0 skip:29 total:65

 $ $CMD CONFIG_GCC_PLUGIN_STRUCTLEAK_USER=y | grep stackinit:
 # stackinit: pass:37 fail:0 skip:28 total:65

 $ $CMD CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF=y | grep stackinit:
 # stackinit: pass:55 fail:0 skip:10 total:65

 $ $CMD CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y | grep stackinit:
 # stackinit: pass:62 fail:0 skip:3 total:65

 $ $CMD CONFIG_INIT_STACK_ALL_PATTERN=y --make_option LLVM=1 | grep stackinit:
 # stackinit: pass:60 fail:0 skip:5 total:65

 $ $CMD CONFIG_INIT_STACK_ALL_ZERO=y --make_option LLVM=1 | grep stackinit:
 # stackinit: pass:60 fail:0 skip:5 total:65

Temporarily remove the userspace-build mode, which will be restored in a
later patch.

Expand the size of the pre-case switch variable so it doesn't get
accidentally cleared.

Cc: David Gow <davidgow@google.com>
Cc: Daniel Latypov <dlatypov@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v1: https://lore.kernel.org/lkml/20220224055145.1853657-1-keescook@chromium.org
v2:
 - split "userspace KUnit stub" into separate header and patch (Daniel)
 - Improve commit log and comments (David)
 - Provide mapping of expected XFAIL tests to CONFIGs (David)