tests: gstdatetime: move gst_date_time_new* and time() calls closer

While extremelly rare, time and gst_date_time_new_* will have
diff values and potentially trigger an assertion. Thus move
the calls as closely together as possible to mitigate this.
8 jobs for alatiera/datetime-test-race in 26 minutes and 52 seconds (queued for 3 minutes and 7 seconds)
Status Job ID Name Coverage
  Preparation
passed #78239
gst indent

00:00:30

passed #78238
manifest

00:00:20

 
  Build
passed #78240
build fedora x86_64

00:03:18

 
  Test
passed #78241
check fedora

00:03:26

 
  Full Builds
passed #78242
build android arm64 api28

00:03:48

passed #78244
build cerbero cross win32

00:06:29

passed #78245
build cerbero cross win64

00:06:58

passed #78243
build cerbero fedora x86_64

00:07:27