btrace, testsuite: fix extended-remote fail

Parts of gdb.btrace/enable.exp are only valid for native debug.  The check for
skip_gdbserver_tests is done while GDB is running, though, which causes it to
fail with --target_board=native-extended-gdbserver.  Exit GDB before that check.

testsuite/
	* gdb.btrace/enable.exp: Call gdb_exit before skip_gdbserver_tests.