[gdb/testsuite] Fix check-read1 FAILs in mi-fortran-modules.exp

When running test-case gdb.mi/mi-fortran-modules.exp with check-read1, I run
into:
...
FAIL: gdb.mi/mi-fortran-modules.exp: -symbol-info-module-functions (timeout)
FAIL: gdb.mi/mi-fortran-modules.exp: -symbol-info-module-functions \
  --name _all (unexpected output)
FAIL: gdb.mi/mi-fortran-modules.exp: -symbol-info-module-functions \
  --module mod[123] (unexpected output)
FAIL: gdb.mi/mi-fortran-modules.exp: -symbol-info-module-variables \
  (unexpected output)
...

Fix this by using exp_continue.

Tested on x86_64, using make target check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-14  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/mi-fortran-modules.exp: Use exp_continue.