feat: add support for RUST_TESTDIR environment variable

[Frando]

This adds support for a RUST_TESTDIR environment variable to override the testdir location.
This can be useful if you want to manually "name" specific test runs to compare results.:

RUST_TESTDIR=/tmp/testdir-main cargo test
git checkout my-branch
RUST_TESTDIR=/tmp/testdir-branch cargo test

It also becomes useful when using with other tools that might orchestrate concurrent runs of tests, and thus don't have to parse out the testdir-x structure if they want to use or inspect the results.