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.