[lit] Add builtin support for flaky tests in lit

This commit adds a new keyword in lit called ALLOW_RETRIES. This keyword
takes a single integer as an argument, and it allows the test to fail that
number of times before it first succeeds.

This work attempts to make the existing test_retry_attempts more flexible
by allowing by-test customization, as well as eliminate libc++'s FLAKY_TEST
custom logic.

Differential Revision: https://reviews.llvm.org/D76288