Migrate the published test base classes off PlexusTestCase

[slachiewicz]

Backport of #936. Same four published base classes: WagonTestCase,
CommandExecutorTestCase, HttpWagonTestCase and KnownHostsProviderTestCase.

This one reaches third-party providers, and wagon-3.x is a patch line. These classes live
in src/main/java of released artifacts and carry no annotations today — they are discovered by
JUnit 3 naming through PlexusTestCase extends junit.framework.TestCase. A provider whose test
does class MyWagonTest extends HttpWagonTestCase {} inherits those tests today; once the base
is on Jupiter and that provider has no Jupiter engine in their own POM, surefire keeps selecting
the JUnit 4 provider and the inherited tests are no longer discovered — a green build with
no coverage, rather than a compile error. Their own testXxx() methods, written in the style
this base class taught them, stop running for the same reason. This is worth a release note.

The earlier plan kept wagon-3.x on JUnit 4 for exactly that reason; the decision to accept it
here is deliberate.

Three things carried over from #936, unchanged: plexus-testing is compile scope on
wagon-provider-test and wagon-ssh-common-test because @PlexusTest sits on published
classes; the six members lost with PlexusTestCase are replaced rather than dropped so
subclasses keep compiling; and runTest()/testSkipped become assumeTrue, so wagon-scm tests
needing an external cvs now report as skipped where they previously reported as passing.

Five places this branch deliberately differs from master, because the lines have drifted:

• Assertions, HttpWagonTests and StreamingWagonTestCase keep plexus-util's FileUtils
  and IOUtil. Master dropped them for java.nio; here they are still in use and unrelated to
  the migration.
• HttpWagonTestCase keeps the commented-out testDeflateGet block that master has since
  removed.
• HttpWagonTestCase keeps FileUtils.fileRead rather than taking master's switch to
  Files.readAllBytes; only the assertEquals argument order changes.
• AbstractHttpClientWagonTest swaps four assertThat(x, instanceOf(Y.class)) for
  assertInstanceOf. Hamcrest was never declared here — it arrived transitively through
  junit:junit, which this change removes. Master has no Hamcrest left to worry about; on this
  branch the choice was between declaring hamcrest-core:1.3 explicitly or using the assertion
  JUnit 5 already provides.
• FtpWagonTest gains a static import for assertTrue, which it previously inherited from
  junit.framework.TestCase. Master replaced that call with assertNotNull(e.getMessage()) in
  unrelated work; that improvement is not backported here.

Verified: per-module <testcase> element counts in the surefire XML are unchanged against
3fca786f across all eight provider modules, with no new errors or failures.

This change was created with AI assistance.