[gdb/testsuite] Fix internal buffer full error in gdb.fortran/module.exp

[gdb/testsuite] Fix internal buffer full error in gdb.fortran/module.exp

With test-case gdb.fortran/module.exp, I run into:
...
PASS: gdb.fortran/module.exp: fully qualified name of DW_TAG_constant
ERROR: internal buffer is full.
UNRESOLVED: gdb.fortran/module.exp: info variables -n
...

Fix this by using exp_continue while matching the output of "info variable
-n".

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

gdb/testsuite/ChangeLog:

2020-03-12  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/module.exp: Use exp_continue during matching of output
	of "info variable -n".