Fix MojoExtension.beforeEach to use proper model

[slawekjaranowski]

Note: as discussed during review, there's a pre-existing issue in MojoExtension.beforeEach() — alignToBaseDirectory is called on tmodel (the raw parsed POM) instead of model (the merged model). This means:

1. If no POM is found, tmodel is null and alignToBaseDirectory will NPE
2. The stored model misses the defaults from the merge (groupId, artifactId, build paths, etc.)

This should be addressed in a follow-up.

Originally posted by @gnodet in #11829 (comment)