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>