test: fix fio.test.lua flakiness

In 89c73e64 ("fio: respect
$TMPDIR in fio.tempdir(), when it is set") was added a test
checking that fio.tempdir() returns a path to a folder, stored
by a path specified in $TMPDIR environment variable.

Check was done by calling Lua returned_path:find(tmpdir_path).
If tmpdir path contained 'special' characters such as '.', it
didn't match, because string.find() takes a regular expression,
not just a string.

string.startswith() works fine.

Follow-up #4794