Fix up return codes for tests in tst-ftell-active-handler

The test functions used a variable ret to store failure codes for
individual tests, but the variable was incorrectly used to record
other failure codes too, resulting in overwriting of the tests status.
This is now fixed by making sure that the ret variable is used only
for recording test failures.

	* libio/tst-ftell-active-handler.c (do_ftell_test): Don't mix
	up test status with function return status.
	(do_write_test): Likewise.
	(do_append_test): Likewise.