[gdb/testsuite] Fix gdb.fortran/nested-funcs-2.exp with gdbserver

[gdb/testsuite] Fix gdb.fortran/nested-funcs-2.exp with gdbserver

When running test-case gdb.fortran/nested-funcs-2.exp with target board
native-gdbserver, we have:
...
(gdb) call contains_keyword::subroutine_to_call()^M
(gdb) FAIL: gdb.fortran/nested-funcs-2.exp: src_prefix=0: nest_prefix=1: \
  call contains_keyword::subroutine_to_call()
...

This is caused by the fact that we're trying to match inferior output using
gdb_test.

Fix this by using gdb_test_stdio instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-14  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/nested-funcs-2.exp: Use gdb_test_stdio to test inferior
	output.