gdb/testsuite: Add a missing -wrap in gdb_test_multiple

In commit:

  commit faeb9f13
  Date:   Wed Feb 24 12:50:00 2021 +0000

      gdb/fortran: add support for ASSOCIATED builtin

A test was added that fails to process the trailing gdb prompt inside
a gdb_test_multiple call, this will cause a failure if the tests are
run with READ1=1, or randomly at other times depending on how the
expect buffers are read in.

Fixed by adding a -wrap argument.

gdb/testsuite/ChangeLog:

	* gdb.fortran/associated.exp: Add missing '-wrap' argument.