Remove directory names from gdb.base/batch-exit-status.exp

Remove directory names from gdb.base/batch-exit-status.exp

Adjust gdb.base/batch-exit-status.exp so that test prefixes don't show
directory names for the source scripts passed with -x, to make test
results from different build directories comparable.

gdb/testsuite/ChangeLog:
2019-08-01  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* gdb.base/batch-exit-status.exp: Call test_exit_status with
	prefix argument.
	(test_exit_status): Add prefix argument.