Fix tests generation after upgrading Gradle to 6.5

buildSrc/src/main/kotlin/CommonUtil.kt

@@ -56,7 +56,7 @@ var Project.javaHome: String?
 
 fun Project.generator(fqName: String, sourceSet: SourceSet? = null) = smartJavaExec {
     classpath = (sourceSet ?: testSourceSet).runtimeClasspath
-    main = fqName
+    mainClass.set(fqName)
     workingDir = rootDir
     systemProperty("line.separator", "\n")
 }

buildSrc/src/main/kotlin/SmartJavaExec.kt

@@ -39,10 +39,9 @@ fun JavaExec.passClasspathInJar() {
     }
 
     dependsOn(jarTask)
+    mainClass.set("-jar")
 
     doFirst {
-        main = "-jar"
-
         classpath = project.files()
         args = listOf(jarTask.outputs.files.singleFile.path) + args.orEmpty()
     }

buildSrc/src/main/kotlin/localDependencies.kt

@@ -165,7 +165,7 @@ fun Project.runIdeTask(name: String, ideaPluginDir: File, ideaSandboxDir: File,
 
         classpath = mainSourceSet.runtimeClasspath
 
-        main = "com.intellij.idea.Main"
+        mainClass.set("com.intellij.idea.Main")
 
         workingDir = File(intellijRootDir(), "bin")