[sdk-platform-java] Some downstream native configs initialize JUnit classes at build time in production

From https://github.com/googleapis/sdk-platform-java/pull/3674

Some libraries mark JUnit classes for initialization at build time in its _production_ native properties file.

For example: https://github.com/googleapis/java-spanner-jdbc/blob/fa65ead20018c083749bb1bd1cc1993f48745ef9/src/main/resources/META-INF/native-image/com.google.cloud/google-cloud-spanner-jdbc/native-image.properties#L15

I tried moving it to a test-only config file (`src/test/resources...`) but the image creation would fail asking to specify the JUnit classes to be initialized at runtime.

Start with the downstream native-image.properties example in java-spanner-jdbc and compare its production configuration with the proposed src/test/resources configuration. Investigate why image creation still requires JUnit classes at runtime, and consider the issue resolved when production configs no longer initialize JUnit classes at build time without breaking test image creation.

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