[llvm] [compiler-rt] follow-up to 166837, rename COMPILER_RT_FORCE_TEST_BUILTINS_DIR to COMPILER_RT_TEST_BUILTINS_DIR (PR #171741)

[Wael Yehia via llvm-commits]

w2yehia wrote:

Seems the [intended final name](https://github.com/llvm/llvm-project/pull/166837#discussion_r2597418972) is `COMPILER_RT_TEST_BUILTINS_DIR`

https://github.com/llvm/llvm-project/pull/171741