tests: Fix timezone inconsistency in squashfs_test

tests: Fix timezone inconsistency in squashfs_test

The image timestamp was not returned in UTC, but the following logic
expected and used UTC.

This patch fixes the test failure like described below:

  unsquashfs -s /tmp/grub-fs-tester.20230407111703613257436.squash4_gzip.9R4/squash4_gzip_512_4096_1_0.img
  grep '^Creation'
  awk '{print $6 " " $7 " " $8 " " $9 " " $10; }'
  FSTIME='Fri Apr 7 11:17:05 2023'
  date -d 'Fri Apr 7 11:17:05 2023' -u '+%Y-%m-%d %H:%M:%S'
  FSTIME='2023-04-07 11:17:05'
  date -d '2023-04-07 11:17:05 UTC -1 second' -u '+%Y-%m-%d %H:%M:%S'
  FSTIMEM1='2023-04-07 11:17:04'
  date -d '2023-04-07 11:17:05 UTC -2 second' -u '+%Y-%m-%d %H:%M:%S'
  FSTIMEM2='2023-04-07 11:17:03'
  date -d '2023-04-07 11:17:05 UTC -3 second' -u '+%Y-%m-%d %H:%M:%S'
  FSTIMEM3='2023-04-07 11:17:02'
  grep -F 'Last modification time 2023-04-07 11:17:05'
  echo 'Device loop0: Filesystem type squash4 - Last modification time 2023-04-07 03:17:05 Friday - Sector size 512B - Total size 10680KiB'
  echo 'Device loop0: Filesystem type squash4 - Last modification time 2023-04-07 03:17:05 Friday - Sector size 512B - Total size 10680KiB'
  grep -F 'Last modification time 2023-04-07 11:17:04'
  echo 'Device loop0: Filesystem type squash4 - Last modification time 2023-04-07 03:17:05 Friday - Sector size 512B - Total size 10680KiB'
  grep -F 'Last modification time 2023-04-07 11:17:03'
  echo 'Device loop0: Filesystem type squash4 - Last modification time 2023-04-07 03:17:05 Friday - Sector size 512B - Total size 10680KiB'
  grep -F 'Last modification time 2023-04-07 11:17:02'
  echo FSTIME FAIL

Signed-off-by: Xiaotian Wu <wuxiaotian@loongson.cn>
Reviewed-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>