Rename populateArtifactURLs task and tests

[vjovanov]

Summary

• rename the populateArtifactURLs task implementation class from SearchArtifactURLsTask to PopulateArtifactURLs
• update the Gradle task registration and test references to the renamed task class
• rename the focused test class/file to PopulateArtifactURLsTests

Testing

• ./gradlew :tck-build-logic:test --tests org.graalvm.internal.tck.harness.tasks.SearchArtifactURLsTaskTests
• ./gradlew :tck-build-logic:test --tests org.graalvm.internal.tck.harness.tasks.PopulateArtifactURLsTests