[gdb/testsuite] Fix gdb.cp/cplusfuncs.exp with check-read1

When running check-read1, we run into:
...
FAIL: gdb.cp/cplusfuncs.exp: info function for "operator=(" (timeout)
...

Fix this by using using gdb_test_lines in info_func_regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-10  Tom de Vries  <tdevries@suse.de>

	* gdb.cp/cplusfuncs.exp (info_func_regexp): Use gdb_test_lines.
2 files changed