Fix JUnit 5 migration in org.eclipse.ui.tests.pluginchecks

Add missing JUnit platform imports to MANIFEST.MF that are required
for JUnit 5 test execution. Without these imports, the test framework
fails with:
  java.util.ServiceConfigurationError: org.junit.platform.engine.TestEngine:
  org.junit.jupiter.engine.JupiterTestEngine not a subtype

Added imports:
- org.junit.platform.commons.function;version="[1.14.0,2.0.0)"
- org.junit.platform.suite.api;version="[1.14.0,2.0.0)"

These are standard imports required by all JUnit 5 test bundles in the
platform.ui repository. Verified by comparing with other successfully
migrated test bundles like org.eclipse.jface.tests.

All 3 tests now pass:
- validateAccessToBundle
- ensurePluginxmlContainsAtLeastOneEntry
- ensureExtensionPointClassesAreAccessable

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: vogella

tests/org.eclipse.ui.tests.pluginchecks/META-INF/MANIFEST.MF

@@ -29,4 +29,6 @@ Bundle-ActivationPolicy: lazy
 Bundle-Vendor: Eclipse.org
 Import-Package: org.eclipse.ui.internal,
  org.junit.jupiter.api;version="[5.14.0,6.0.0)",
- org.junit.jupiter.api.function;version="[5.14.0,6.0.0)"
+ org.junit.jupiter.api.function;version="[5.14.0,6.0.0)",
+ org.junit.platform.commons.function;version="[1.14.0,2.0.0)",
+ org.junit.platform.suite.api;version="[1.14.0,2.0.0)"