gdb/testsuite: Avoid a buffer overrun in `gdb.base/maint.exp'

gdb/testsuite: Avoid a buffer overrun in `gdb.base/maint.exp'

Fixes:

 PASS: gdb.base/maint.exp: maint w/o args
 ERROR: internal buffer is full.
 UNRESOLVED: gdb.base/maint.exp: maint info line-table w/o a file name

The problem is just many symtabs and long line tables, enough to
overflow the expect buffer.  Fix this by matching input incrementally.

gdb/testsuite/ChangeLog:
2016-10-28  Pedro Alves  <palves@redhat.com>

	* gdb.base/maint.exp <maint info line-table w/o a file name>: Use
	gdb_test_multiple, tighten regexps and match symtabs and line
	tables incrementally.