[leoweigl]

This adds the first unit tests to the project and fixes the test runtime so ./gradlew test works.

Test runtime fix

With the first test under src/test/java, ./gradlew test failed before running any test:
NoClassDefFoundError: org/junit/runners/model/Statement.
The IntelliJ Platform test framework refers to JUnit 4 classes even with JUnit 5
(see the IntelliJ Platform Gradle Plugin FAQ,
"JUnit5 Test Framework refers to JUnit4"). I added junit:junit:4.13.2 as testRuntimeOnly,
so it is only on the runtime classpath and cannot be used by test code.

Tests (ListenerTest)

• IDs are unique (they are persisted in the user settings, a duplicate would silently map to the wrong listener)
• fromId returns the matching listener for every known ID
• fromId returns DEFAULT for unknown IDs (parameterized)

Comments I left unchanged

Two comments in build.gradle.kts are slightly outdated now, I did not want to change them without your opinion:

• above testIdeUi: "adding the first test under src/test/java is all it takes" (the JUnit 4 dependency was needed as well)
• above instrumentTestCode: "Pointless without test sources" (there are test sources now, the task stays disabled
  because of the InstrumentCodeTask crash; tests run fine with it disabled)

Tested locally on Windows with JDK 21 (./gradlew test, 6 tests green).

[0verEngineer]

Thanks, looks good! Please update the three outdated comments in this PR:

1. build.gradle.kts, above testIdeUi: "adding the first test under src/test/java is all it takes" no longer holds, since the JUnit 4 runtime dependency was needed too. "runs an empty suite" is also outdated now that there are tests under src/test.
2. build.gradle.kts, above instrumentTestCode: "Pointless without test sources" is outdated now. The task stays disabled only because of the InstrumentCodeTask crash.
3. .github/workflows/run-ui-tests.yml, header: it has the same outdated note ("no UI tests yet … empty suite … adding the first test under src/test is enough") and should be adjusted the same way as 1.

[leoweigl]

Updated all three comments, thanks!