[libc++] Simplify how we install test-suite dependencies (#171504)

Based on comments in #171474, it was brought to my attention that we can
modernize and simplify how we perform the test suite installation in
libc++ and libc++abi.

NOKEYCHECK=True
GitOrigin-RevId: f582fc649178931694d6e360ec79e8c1fe8a0d0b