tests: datetime: Fix failure on Windows

The documentation for WIN32 mktime indicates that for struct tm*
before January 1, 1970, that -1 is returned, and since mktime is timezone
dependent, the struct tm corresponding to 1:00, Jan. 1, 1970 might be failed.

See also
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/mktime-mktime32-mktime64
6 jobs for fix-datetime in 33 minutes and 2 seconds (queued for 57 minutes and 33 seconds)
Status Job ID Name Coverage
  Preparation
passed #65559
gst indent

00:00:30

passed #65558
manifest

00:00:21

 
  Build
passed #65560
build fedora x86_64

00:03:00

 
  Test
passed #65561
check fedora

00:15:34

 
  Full Builds
passed #65562
build android arm64 api28

00:03:18

passed #65563
build cerbero fedora x86_64

00:10:18