[jkschneider]

The Code Genome Project builds this repository with MAVEN_CENTRAL_MIRROR_URL, _USERNAME and _PASSWORD naming a mirror, and Maven settings that mirror everything through it. Since #206, two tests there still reached repo.maven.apache.org directly, one connection each.

• DependencyListTest#validateResolvable gives the recipe an LST whose only repository, https://nonexistent, can't be reached. Its xml() source loads no Maven settings, and the downloader adds Maven Central behind that repository, so the test asked Central for com/test/test/maven-metadata.xml and its directory listing, and got 404s. Its context now leaves out Central and the local repository, so the pretend repository is the only one asked. The recipe still reports the MavenDownloadingException the test checks for.
• ModuleHasDependencyTest.WhenDependencyIsRequestedButNotResolved#mavenMatchesOnRequested forced empty settings, which keep the machine's own out but name no mirror, so spring-beans 6.0.0 and the POMs it needs came from Central directly. Like DependencyResolutionDiagnosticTest#maven since Check the diagnostic's report without naming Maven Central's or the plugin portal's URL #206, it now uses settings that mirror only Central through MAVEN_CENTRAL_MIRROR_URL when that is set, and the same empty settings when it is not. settingsMirroringOnlyCentral moves from DependencyResolutionDiagnosticTest to MavenCentralMirror, unchanged, so that both tests can use it.

Locally, against the Code Genome Project's current snapshots, DependencyListTest, ModuleHasDependencyTest and DependencyResolutionDiagnosticTest pass with and without MAVEN_CENTRAL_MIRROR_* set, 48 tests each. I also recorded every socket read and write of the test JVMs with Java Flight Recorder while those classes ran with the mirror set:

• On main, validateResolvable and mavenMatchesOnRequested each opened a connection to repo.maven.apache.org.
• With this change, every read and write the three classes make to another machine goes to the mirror. validateResolvable makes none at all.

With no mirror named, mavenMatchesOnRequested still reaches Central directly, as before.