gdb/testsuite: Remove duplicates from gdb.base/decl-before-def.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/decl-before-def.exp ...
    DUPLICATE: gdb.base/decl-before-def.exp: p a

Fix by giving explicit names to the two tests that use the same command.

Tested on x86_64-linux.
1 file changed