twister: testcases skipped by ztest_test_skip() have reason "Unknown" in report

[gopiotr]

Describe the bug
When testcase written in ztest framework is skipped by ztest_test_skip() function, next it is placed in report with status "Unknown". For example in source code of test: zephyr/tests/kernel/common/src/main.c it can be observed, that testcase test_bitfield will be skipped on ARM boards:

#ifdef CONFIG_ARM
void test_bitfield(void)
{
	ztest_test_skip();
}
#else
extern void test_bitfield(void);
#endif

As result after perform this test on some ARM board (for example nrf52840dk_nrf52840, it can be observed following entry in twister_report.xml or twister.json:

<testcase classname="kernel.common" name="kernel.common.bitfield" time="7.675848">
<skipped type="skipped" message="Unknown"/>
</testcase>

{
    "testcase":"kernel.common.bitfield",
    "arch":"arm",
    "platform":"nrf52840dk_nrf52840",
    "ram_size":29433,
    "rom_size":122406,
    "status":"skipped",
    "reason":"Unknown"
},

To Reproduce
Steps to reproduce the behavior:

1. Run command:
   scripts/twister --device-testing --device-serial /dev/ttyACM0 --json-report -p nrf52840dk_nrf52840 -T tests/kernel/common/
2. Observe results in twister_report.xml or twister.json reports, especially entry for testcase kernel.common.bitfield

Expected behavior
Reason of performed skip should be more readable, because in this situation it is known reason of this skip, for example it should be like this:

<testcase classname="kernel.common" name="kernel.common.bitfield" time="7.675848">
<skipped type="skipped" message="Testcase skipped by ztest_test_skip() function in test's source code."/>
</testcase>

Impact
Report with such skip is suspicious, because it is not clear if this skip is performed as intentional skip in test's source code or if it is made by mistake/bug.

Environment (please complete the following information):

• OS: Linux
• Toolchain: zephyr-sdk-0.13.1
• Commit SHA: efa170f

Additional context
I will crate PR with suitable fix soon.