gdb/testsuite: Remove duplicates from gdb.base/dfp-test.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/dfp-test.exp ...
    DUPLICATE: gdb.base/dfp-test.exp: 1.23E is an invalid number
    DUPLICATE: gdb.base/dfp-test.exp: 1.23E45A is an invalid number
    DUPLICATE: gdb.base/dfp-test.exp: 1.23E is an invalid number
    DUPLICATE: gdb.base/dfp-test.exp: 1.23E45A is an invalid number

Fix by using proc_with_prefix where appropriate.

Tested on x86_64-linux.
Co-Authored-By: Andrew Burgess <aburgess@redhat.com>