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") {

app/src/test/kotlin/com/subtracks/TestSupport.kt

@@ -4,7 +4,7 @@ import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.cancelAndJoin
 import kotlinx.coroutines.runBlocking
-import org.junit.Assert.assertTrue
+import org.junit.Assert.fail
 
 const val TEST_TIMEOUT_MS = 30_000L
 
@@ -18,5 +18,14 @@ fun awaitUntil(
 ) {
     val deadline = System.nanoTime() + TEST_TIMEOUT_MS * 1_000_000
     while (!predicate() && System.nanoTime() < deadline) Thread.sleep(10)
-    assertTrue("Timed out after ${TEST_TIMEOUT_MS}ms $message", predicate())
+    if (predicate()) return
+    // A wall-clock wait that expires means something stalled rather than ran slow: the suite
+    // finishes in milliseconds per test. Print every thread's stack in the failure message so the
+    // stall is diagnosable from the CI log, which shows exception messages but not the test XML.
+    fail("Timed out after ${TEST_TIMEOUT_MS}ms $message\n\n${threadDump()}")
 }
+
+private fun threadDump(): String =
+    Thread.getAllStackTraces().entries.joinToString("\n\n") { (thread, frames) ->
+        "${thread.name} [${thread.state}]\n" + frames.joinToString("\n") { "    at $it" }
+    }