gdb/testsuite: Remove duplicates from gdb.base/subst.exp

When I run the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/subst.ex ...
    DUPLICATE: gdb.base/subst.exp: unset substitute-path from, no rule entered yet

Fix by adjusting the problematic test name.

Tested on x86_64-linux.