Build: implement useBootstrapStdlib flag

Excludes stdlib projects from build and uses bootstrap stdlib artifacts

 #KT-29205

generators/test-generator/build.gradle.kts

@@ -10,7 +10,7 @@ dependencies {
     compile(intellijDep()) { includeJars("util") }
     testCompile(project(":core:util.runtime"))
     testCompile(projectTests(":compiler:tests-common"))
-    testCompile(project(":kotlin-stdlib"))
+    testCompile(kotlinStdlib())
     testCompile(commonDep("junit:junit"))
 }