rtc: test: Test date conversion for dates starting in 1900

While the RTC framework intends to only handle dates after 1970 for
consumers, time conversion must also work for earlier dates to cover
e.g. storing dates beyond an RTC's range_max. This is most relevant for
the rtc-mt6397 driver that has

	range_min = RTC_TIMESTAMP_BEGIN_1900;
	range_max = mktime64(2027, 12, 31, 23, 59, 59);

and so needs working support for timestamps in 1900 starting in less than
three years.

So shift the tested interval of timestamps to also cover years 1900 to
1970.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://lore.kernel.org/r/20250428-enable-rtc-v4-5-2b2f7e3f9349@baylibre.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>