Replace deprecated %T in 2 tests.

In D91442, @MaskRay commented about a failure. This commit does the following to
address his comments:

1. Replace %T with %t as former is deprecated.
2. Add an explicit --sysroot argument in a test.

Some tests were failing when gcc-10-riscv64-linux-gnu is installed on test machine.
This was happening because the test was checking a case when --gcc-toolchain is not
provided. But if --sysroot was also not provided then code could pick a toolchain
installed in /usr. So to make the test more robust, I have provided an explicit --sysroot
argument. Its value has been chosen to match the existing patterns.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D93023