Migrate the published test base classes off PlexusTestCase

[slachiewicz]

Completes the JUnit 5 move for the test base classes that wagon publishes: WagonTestCase,
CommandExecutorTestCase, HttpWagonTestCase and KnownHostsProviderTestCase. These are
src/main/java in released artifacts, so the change reaches consumers who extend them —
which drove three decisions worth a reviewer's attention.

plexus-testing is a compile-scope dependency of wagon-provider-test and
wagon-ssh-common-test. @PlexusTest sits on published classes, so a consumer extending
WagonTestCase needs it on their own compile path; test scope would not propagate. Its version
moves to the root dependencyManagement alongside the other third-party versions, since
wagon-ssh already declared it and this change would otherwise make three copies.

Six members disappeared with PlexusTestCase and are replaced rather than dropped, so
subclasses outside this repo keep compiling: getTestFile, getTestPath and getBasedir
delegate to PlexusExtension; getName() is served by a TestInfo callback kept separate from
setUp() — eight subclasses override setUp() and call super.setUp(), and widening that
signature would break every one of them.

runTest() and the testSkipped flag are gone, replaced by assumeTrue. This changes a CI
signal: on a machine without cvs, the old mechanism reported those wagon-scm tests as
passing, and they are now reported as skipped (0 → 14 locally). Same tests, honest status.

Checkstyle FileLength is suppressed for HttpWagonTestCase via a new
src/config/checkstyle-suppressions.xml. One @Test per method took it one line past the
inherited 2000-line limit. Neither @SuppressWarnings nor a CHECKSTYLE_OFF comment can
suppress it — the reasons are in a comment in that file. Splitting the class is a separate change.

Verified: per-module <testcase> element counts in the surefire XML are unchanged against
f289bdd5 across all eight provider modules (22/23/295/214/42/0/0/295), with no errors or
failures.

This change was created with AI assistance.

[slachiewicz]

The wagon-3.x counterpart is #937. It is not a clean lift — 18 of the 35 files had drifted between the lines, so it carries five deliberate deviations, listed in its description.

This comment was created with AI assistance.