Add meson test --interactive

[DaanDeMeyer]

This is very similar to --gdb, except it doesn't spawn GDB, but connects stdin/stdout/stderr directly to the test itself. This allows interacting with integration tests that spawn a shell in a container or virtual machine when the test fails.

In systemd we're migrating our integration tests to run using the meson test runner. We want to allow interactive debugging of failed tests directly in the virtual machine or container that is spawned to run the test. To make this possible, we need meson test to connect stdin/stdout/stderr of the test directly to the user's terminal, just like is done with the --gdb option.

[DaanDeMeyer]

Let me know if I added the release notes snippet in the right file

[eli-schwartz]

docs/markdown/snippets/*.md

See https://mesonbuild.com/Contributing.html#documentation

[DaanDeMeyer]

@jpakkane @eli-schwartz Any chance this could get another look? This is the final piece we need to allow debugging systemd's integration tests.

[jpakkane]

LGTM