On slow systems, the rtc unit test may result in soft lockups and/or
generate messages such as

 # rtc_time64_to_tm_test_date_range: Test should be marked slow (runtime: 34.253230015s)
 # rtc_time64_to_tm_test_date_range: pass:1 fail:0 skip:0 total:1

The test covers a date range of 160,000 years, resulting in the long
runtime.

Unit tests running for more than 1 second are supposed to be marked as
slow. Just marking the test as slow would prevent it from running when
slow tests are disabled, which would not be desirable. At the same time,
the current test range of 160,000 years seems to be of limited value.

Split the test into two parts, one covering a range of 1,000 years and
the other covering the current range of 160,000 years. Mark the 160,000
year test as slow to be able to separate it from the faster test.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240313174221.1999654-1-linux@roeck-us.netSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>