Stop tests from polluting maven settings (#18525) (#18528)

The pluginmanager tests modify `~/.m2/settings.xml`. Previously when that file
exists it would be restored on test cleanup. However if that file did not exist
prior to the tests, the file with nonsense values generated during testing would
persist. This would cause subsequent issues when the pluginmanager or other
systems that respect those maven settings. This commit updates the test to
ensure the test file is deleted if it did not previously exist.

(cherry picked from commit f36ba65)

Co-authored-by: Cas Donoghue <[email protected]>

Author: mergify[bot]

spec/unit/plugin_manager/proxy_support_spec.rb

@@ -30,7 +30,11 @@
   end
 
   after do
-    FileUtils.mv(settings_backup, settings) if File.exist?(settings_backup)
+    if File.exist?(settings_backup)
+      FileUtils.mv(settings_backup, settings)
+    elsif File.exist?(settings)
+      FileUtils.rm(settings)
+    end
     environments.each { |key, _| ENV[key] = nil }
   end