tests: fix metadata tests when SOURCE_DATE_EPOCH is set
    
    SOURCE_DATE_EPOCH is preferred over time.time() call in metadata
    generation, which makes time.time() mock ineffective. Set
    SOURCE_DATE_EPOCH instead of mocking time.time(). This additionally test
    if SOURCE_DATE_EPOCH is really used.
    Adjust expected output, as SOURCE_DATE_EPOCH is defined without
    fractional component.
    
    Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>