fstests: Add path $here before src/<file>

fstests: Add path $here before src/<file>

Calling src/<file> without path '$here' may cause the problem that
the file cannot be found.

For example, Running generic/192 with overlayfs(Let ubifs as base
fs) yields the following output:

  generic/192 - output mismatch
     QA output created by 192
     sleep for 5 seconds
     test
    +./common/rc: line 316: src/t_dir_type: No such file or directory
     delta1 is in range
     delta2 is in range
    ...

When the use case fails, the call stack in generic/192 is:

  local unknowns=$(src/t_dir_type $dir u | wc -l)	common/rc
  _supports_filetype					common/rc
  _overlay_mount					common/overlay
  _overlay_test_mount					common/overlay
  _test_mount						common/rc
  _test_cycle_mount					generic/192

Before _test_cycle_mount() being invoked, generic/192 executed 'cd
/' to change work dir from 'xfstests-dev' to '/', so src/t_dir_type
was not found.

[Eryu: some tests run src/<file> as regular user, don't add $here
prefix in such case, as a regular user may have no search permission
on $here]

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>