check: fix wallclock wrapping problem

check: fix wallclock wrapping problem

If a test cross two days, the time may be negative, let's use
the UTC seconds instead, for example:

2014-04-03 23:43:42 ./check generic/074 generic/075 generic/112 generic/113 generic/132 generic/133 generic/231
FSTYP         -- xfs (non-debug)
PLATFORM      -- Linux/x86_64 vpx-2 3.14.0-00001-g462fa88
MKFS_OPTIONS  -- -f -bsize=4096 /dev/vdd
MOUNT_OPTIONS -- /dev/vdd /fs/scratch

generic/074	 100s
generic/075	 220s
generic/112	 226s
generic/113	 188s
generic/132	 18s
generic/133	 37s
generic/231	 -85401s
Ran: generic/074 generic/075 generic/112 generic/113 generic/132 generic/133 generic/231
Passed all 7 tests

Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>