Implement new 'mix' test mode

This mode must be used when one configuration will run several tests
for different JUnit versions.

plugins/fir/fir-plugin-prototype/build.gradle.kts

@@ -60,7 +60,7 @@ if (kotlinBuildProperties.isInJpsBuildIdeaSync) {
     }
 }
 
-projectTest(parallel = true, jUnit5Enabled = true) {
+projectTest(parallel = true, jUnitMode = JUnitMode.JUnit5) {
     workingDir = rootDir
     jvmArgs!!.removeIf { it.contains("-Xmx") }
     maxHeapSize = "3g"

plugins/kotlin-serialization/kotlin-serialization-compiler/build.gradle.kts

@@ -51,7 +51,7 @@ sourcesJar()
 javadocJar()
 testsJar()
 
-projectTest(parallel = true, jUnit5Enabled = true) {
+projectTest(parallel = true, jUnitMode = JUnitMode.JUnit5) {
     workingDir = rootDir
     useJUnitPlatform()
 }

plugins/parcelize/parcelize-compiler/build.gradle.kts

@@ -75,7 +75,7 @@ sourcesJar()
 
 testsJar()
 
-projectTest(jUnit5Enabled = true) {
+projectTest(jUnitMode = JUnitMode.JUnit5) {
     useJUnitPlatform()
     dependsOn(parcelizeRuntimeForTests)
     dependsOn(":dist")