[test] Use %t instead of %T to remove race conditions between config-file3.c and target-override.c

Both tests operate on `%T/testbin`. If the two tests run concurrently,
one may fail.

This is likely the root cause of flaky failures reported by
https://lists.llvm.org/pipermail/llvm-dev/2020-September/144781.html

https://llvm.org/docs/CommandGuide/lit.html says:

`%T	parent directory of %t (not unique, deprecated, do not use)`

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D87103