fix: JUnit tests don't run when targeting Eclipse 2025-12

When running  `clean verify -Dtm4e.target-platform=staging` Tycho fails
with:
[ERROR] Failed to execute goal
org.eclipse.tycho:tycho-surefire-plugin:5.0.0:test (default-test) on
project org.eclipse.tm4e.core.tests: Could not determine test framework
provider. Available providers: [junit5vintage, junit4,
junit5vintageinternal, junit5, testng]

Author: sebthom

pom.xml

@@ -328,6 +328,7 @@
 						<useUIHarness>true</useUIHarness>
 						<useUIThread>true</useUIThread>
 						<argLine>-Xms512m -Xmx512m -Dfile.encoding=${project.build.sourceEncoding} ${tycho-surefire-plugin.vmargs}</argLine>
+						<providerHint>junit5</providerHint>
 					</configuration>
 				</plugin>
 				<plugin>