test: Various mkosi integration test improvements

test: Various mkosi integration test improvements

- Stop using logging module since the default output formatting is
  pretty bad. Prefer print() for now.
- Log less, logging the full mkosi command line is rather verbose,
  especially when it contains multi-line dropins.
- Streamline the journalctl command we output for debugging failed
  tests.
- Don't force usage of the disk image format.
- Don't force running without unit tests.
- Don't force disabling RuntimeBuildSources.
- Update documentation to streamline the command for running a single
  test and remove sudo as it's not required anymore.
- Improve the console output by having the test unit's output logged
  to both the journal and the console.
- Disable journal console log forwarding as we have journal forwarding
  as a better alternative.
- Delete existing journal file before running test.
- Delete journal files of succeeded tests to reduce disk usage.
- Rename system_mkosi target to just mkosi
- Pass in mkosi source directory explicitly to accomodate arbitrary
  build directory locations.
- Add test interactive debugging if stdout is connected to a tty
- Stop explicitly using the 'system' image since it'll likely be
  dropped soon.
- Only forward journal if we're not running in debugging mode.
- Stop using testsuite.target and instead just add the necessary
  extras to the main testsuite unit via the credential dropin.
- Override type to idle so test output is not interleaved with
  status output.
- Don't build mkosi target by default
- Always add the mkosi target if mkosi is found
- Remove dependency of the integration tests on the mkosi target
  as otherwise the image is always built, even though we configure
  it to not be built by default.
- Move mkosi output, cache and build directory into build/ so that
  invocations from meson and regular invocations share the same
  directories.
- Various aesthetic cleanups.