gdb.gdb/selftest.exp, Use multi_line to build gdb's expected startup output

gdb.gdb/selftest.exp, Use multi_line to build gdb's expected startup output

This regex had to be touched at least twice these past few days.  Use
multi_line to make it more readable.

Note this also tightens the regex a little bit in some spots.

gdb/testsuite/ChangeLog:
2018-06-14  Pedro Alves  <palves@redhat.com>

	* gdb.gdb/selftest.exp (test_with_self): Use multi_line to build
	gdb's expected startup output.