Skip model validation of resolved POMs

[basil]

Chases away the new warnings inadvertently introduced in #310:

[INFO] --- maven-compiler-plugin:3.10.1:testCompile (default-testCompile) @ git-client ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 66 source files to /home/basil/src/jenkinsci/git-client-plugin/target/test-classes
[INFO] 
[INFO] --- maven-hpi-plugin:3.27-SNAPSHOT:test-hpl (default-test-hpl) @ git-client ---
[INFO] Generating /home/basil/src/jenkinsci/git-client-plugin/target/test-classes/the.hpl
[INFO] 
[INFO] --- maven-hpi-plugin:3.27-SNAPSHOT:resolve-test-dependencies (default-resolve-test-dependencies) @ git-client ---
[INFO] 
[INFO] --- gmavenplus-plugin:1.13.1:compileTests (test-in-groovy) @ git-client ---
[INFO] No sources specified for compilation. Skipping.

Fixes #325.

[MarkEWaite]

No objection from me, but I'm not a credible reviewer of maven plugin changes.

I assume that changing the model validation level will change the messages being output but should not change the results of the goal being executed.

[jglick]

Good idea. In this context we do not care about validation warnings. Might help with some other irrelevant noise as well.