[gdb/testsuite] Fix duplicates in gdb.fortran/array-{indices,repeat}.exp

[gdb/testsuite] Fix duplicates in gdb.fortran/array-{indices,repeat}.exp

When running test-case gdb.fortran/array-indices.exp on a system without
fortran compiler, I run into a duplicate:
...
Running /home/vries/gdb/src/gdb/testsuite/gdb.fortran/array-indices.exp ...
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/array-indices.exp: array-indices.exp
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/array-indices.exp: array-indices.exp
DUPLICATE: gdb.fortran/array-indices.exp: array-indices.exp
...

Fix this by adding a with_test_prefix at the toplevel.

Likewise in gdb.fortran/array-repeat.exp.

Tested on x86_64-linux.

Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>