Fix batch exit status test failure on Fedora 28

Fix batch exit status test failure on Fedora 28

This commit adds calls to remote_close and clear_gdb_spawn_id to
gdb.base/batch-exit-status.exp, fixing failures reported by buildbot
on Fedora 28 where gdb_spawn_id not being reset by the previous test
caused default_gdb_spawn to return without spawning.

This commit also changes the test to use detect GDB's exit using
gdb_test_multiple expecting 'eof', rather than using 'wait -i' alone.
This means the testcase won't hang forever on failure as fixed in
gdb.base/quit.exp by commit 15763a09d4ae ("Fix 'gdb.base/quit.exp
hangs forever' if the test fails").

gdb/testsuite/ChangeLog:

	* gdb.base/batch-exit-status.exp: Use gdb_test_multiple and expect
	'eof' before 'wait -i'.  Use remote_close and clear_gdb_spawn_id.