The first field of /proc/uptime relies on the CLOCK_BOOTTIME clock which
can also be fetched from clock_gettime() API.

Improve the test coverage while verifying the monotonicity of
CLOCK_BOOTTIME accross both interfaces.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230222144649.624380-9-frederic@kernel.org