[Test] Add ability to set maximum of working threads for parallel JUnit 5 tests

compiler/tests-common-new/testResources/junit-platform.properties

@@ -1,2 +1,5 @@
 junit.jupiter.execution.parallel.enabled=true
 junit.jupiter.execution.parallel.mode.default=concurrent
+junit.jupiter.execution.parallel.config.strategy=custom
+junit.jupiter.execution.parallel.config.custom.class=org.jetbrains.kotlin.test.DynamicWithMaxThresholdParallelExecutionConfigurationStrategy
+junit.jupiter.execution.parallel.config.fixed.threshold=16

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/DynamicWithMaxThresholdParallelExecutionConfigurationStrategy.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.test
+
+import org.junit.platform.engine.ConfigurationParameters
+import org.junit.platform.engine.support.hierarchical.ParallelExecutionConfiguration
+import org.junit.platform.engine.support.hierarchical.ParallelExecutionConfigurationStrategy
+
+class DynamicWithMaxThresholdParallelExecutionConfigurationStrategy : ParallelExecutionConfigurationStrategy {
+    companion object {
+        // Full property name is junit.jupiter.execution.parallel.config.fixed.threshold
+        private const val FIXED_THRESHOLD = "fixed.threshold"
+        private const val DEFAULT_VALUE = 16
+
+        // Copied from org.junit.platform.engine.support.hierarchical.DefaultParallelExecutionConfigurationStrategy
+        private const val KEEP_ALIVE_SECONDS = 30
+    }
+
+    override fun createConfiguration(configurationParameters: ConfigurationParameters): ParallelExecutionConfiguration {
+        val threshold = configurationParameters[FIXED_THRESHOLD].map { it.toIntOrNull() }.orElse(null) ?: DEFAULT_VALUE
+        val cpuCores = Runtime.getRuntime().availableProcessors()
+        val parallelism = if (threshold > 0) minOf(cpuCores, threshold) else cpuCores
+        return object : ParallelExecutionConfiguration {
+            override fun getParallelism(): Int = parallelism
+            override fun getMinimumRunnable(): Int = parallelism
+            override fun getMaxPoolSize(): Int = 256 + parallelism
+            override fun getCorePoolSize(): Int = parallelism
+            override fun getKeepAliveSeconds(): Int = KEEP_ALIVE_SECONDS
+        }
+    }
+}