[gdb/testsuite] Fix corefile-buildid.exp with check-read1

When running gdb.base/corefile-buildid.exp using check-read1, I run into:
...
FAIL: gdb.base/corefile-buildid.exp: shared: info files (timeout)
FAIL: gdb.base/corefile-buildid.exp: symlink shared: info files (timeout)
FAIL: gdb.base/corefile-buildid.exp: shared sepdebug: info files (timeout)
FAIL: gdb.base/corefile-buildid.exp: symlink shared sepdebug: info files \
  (timeout)
...

This is caused by attempting to match the output of an "info files" command
using a single gdb_test in check_exec_file.

Fix this by doing line-by-line matching in check_exec_file.

Tested on x86_64-linux, using make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-02-19  Tom de Vries  <tdevries@suse.de>

	* gdb.base/corefile-buildid.exp (check_exec_file): Match info files
	output line-by-line.