build: stop test results being restored from the build cache

org.gradle.caching=true plus CI restoring ~/.gradle/caches (which holds
the local build cache) let GitHub CI restore :app:testDebugUnitTest
FROM-CACHE: a re-run reported green without executing a single test. A
cached PASS is also unsound while any test is flaky. Mark test tasks
uncacheable so every invocation actually runs the tests; up-to-date and
incremental behaviour for local dev is unchanged.

app/build.gradle.kts

@@ -189,6 +189,7 @@ tasks.withType<Test>().configureEach {
     testLogging {
         exceptionFormat = TestExceptionFormat.FULL
     }
+    outputs.cacheIf { false }
 }
 
 tasks.register<Test>("integrationTest") {