Fix gdb.fortran/nested-funcs.exp failure

Commit 87d6a7aa ("Add DWARF index cache") broke
gdb.fortran/nested-funcs.exp.  Because of the new "set index-cache"
command, the expression "set index = 42" now fails:

  set index = 42
  Undefined set index-cache command: "= 42".  Try "help set index-cache".
  (gdb) PASS: gdb.fortran/nested-funcs.exp: set index = 42

Fix it by changing it to "set variable index = 42".  Also, use
gdb_test_no_output to confirm that it worked (since that particular test
wrongfully passed).

gdb/testsuite/ChangeLog:

	* gdb.fortran/nested-funcs.exp: Replace "set index = 42" with
	"set variable index = 42".