KTIJ-650 [Code completion]: test framework fix

This commit fixes test infrastructure issue.
Usage of "COMPILER_ARGUMENTS" test-data-instruction resulted in side
effect. Test cases following the one that used it got broken
LanguageVersionSetting - LanguageFeature.MultiPlatformProjects escaped,
languageVersion could be wrong.

Why it happened

KotlinProjectDescriptorWithFacet defines default values
of (language-version, isMultiplatform) settings for the test-case.
The values themselves are stored in KotlinFacetSettings and passed there
only once. After every test-case (if it uses "COMPILER_ARGUMENTS")
infrastructure calls
KotlinLightCodeInsightFixtureTestCaseKt#rollbackCompilerOptions which
resets mentioned values (among others) in KotlinFacetSettings.
Instances of KotlinProjectDescriptorWithFacet are reused hence facet
settings remained reset.

idea/idea-completion/tests/org/jetbrains/kotlin/idea/completion/test/AbstractKeywordCompletionTest.kt

@@ -23,9 +23,9 @@ abstract class AbstractKeywordCompletionTest : KotlinFixtureCompletionBaseTestCa
     }
 
     override fun getProjectDescriptor(): KotlinLightProjectDescriptor = when {
-        "LangLevel10" in fileName() -> KotlinProjectDescriptorWithFacet.KOTLIN_10
-        "LangLevel11" in fileName() -> KotlinProjectDescriptorWithFacet.KOTLIN_11
-        else -> KotlinProjectDescriptorWithFacet.KOTLIN_STABLE_WITH_MULTIPLATFORM
+        "LangLevel10" in fileName() -> KotlinProjectDescriptorWithFacet.KOTLIN_10.apply { replicateToFacetSettings() }
+        "LangLevel11" in fileName() -> KotlinProjectDescriptorWithFacet.KOTLIN_11.apply { replicateToFacetSettings() }
+        else -> KotlinProjectDescriptorWithFacet.KOTLIN_STABLE_WITH_MULTIPLATFORM.apply { replicateToFacetSettings() }
     }
 
     override fun defaultInvocationCount() = 1

idea/idea-test-framework/test/org/jetbrains/kotlin/idea/test/KotlinProjectDescriptorWithFacet.kt

@@ -30,13 +30,25 @@ class KotlinProjectDescriptorWithFacet(
     private val languageVersion: LanguageVersion,
     private val multiPlatform: Boolean = false
 ) : KotlinLightProjectDescriptor() {
+
+    private var facetConfig: KotlinFacetConfiguration? = null
+
     override fun configureModule(module: Module, model: ModifiableRootModel, contentEntry: ContentEntry) {
         configureKotlinFacet(module) {
-            settings.languageLevel = languageVersion
-            if (multiPlatform) {
-                settings.compilerSettings = CompilerSettings().apply {
-                    additionalArguments += " -Xmulti-platform"
-                }
+            facetConfig = this
+            toFacetConfig(this)
+        }
+    }
+
+    fun replicateToFacetSettings() {
+        facetConfig?.let { toFacetConfig(it) }
+    }
+
+    private fun toFacetConfig(configuration: KotlinFacetConfiguration) {
+        configuration.settings.languageLevel = languageVersion
+        if (multiPlatform) {
+            configuration.settings.compilerSettings = CompilerSettings().apply {
+                additionalArguments += " -Xmulti-platform"
             }
         }
     }