clock-util: make clock_is_localtime() testable and add initial tests

clock-util: make clock_is_localtime() testable and add initial tests

Add path argument to clock_is_localtime() and default to "/etc/adjtime" if it's
NULL. This makes the function testable.

Add test-clock: initial test cases for some scenarios, using a temporary file.
This also checks the behaviour with a NULL (i. e. the system's /etc/adjtime)
file.