[lldb] make lit use the same Python executable for building and testing (#143756)

When testing LLDB, we want to make sure to use the same Python as the
one we used to build it.

This patch uses the CMake variable `Python3_ROOT_DIR` to add the correct
Python to the `PATH` in LLDB lit tests, in order to ensure of this.

Please see https://github.com/swiftlang/swift/pull/82063 for the
original issue.

This is a continuation of https://github.com/swiftlang/swift/pull/82063.

GitOrigin-RevId: 556e69b7f4328a0d7c36c9d7ca0dd8f52f82ad71