[lldb/Reproducers] Add test-specific API to set the test CWD

The reproducers' working directory is set to the current working
directory when they are initialized. While this is not optimal, as the
cwd can change during a debug session, it has been sufficient so far.

The current approach doesn't work for the API test suite however because
dotest temporarily changes the directory to where the test's Python file
lives.

This patch adds an API to tell the reproducers what to set the CWD to.
This is a NO-OP in every mode but capture.

Differential revision: https://reviews.llvm.org/D79825