Give slightly better names to language/API version variables in FP tests

compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/FE1FullPipelineModularizedTest.kt

@@ -11,7 +11,7 @@ class FE1FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
     override fun configureArguments(args: K2JVMCompilerArguments, moduleData: ModuleData) {
         args.useK2 = false
         args.jvmDefault = "compatibility"
-        args.apiVersion = LANGUAGE_VERSION
+        args.apiVersion = API_VERSION
         args.optIn = arrayOf(
             "kotlin.RequiresOptIn",
             "kotlin.contracts.ExperimentalContracts",

compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/FullPipelineModularizedTest.kt

@@ -7,15 +7,20 @@ package org.jetbrains.kotlin.fir
 
 import org.jetbrains.kotlin.cli.common.arguments.K2JVMCompilerArguments
 
-internal val LANGUAGE_VERSION: String = System.getProperty("fir.bench.language.version", "1.4")
+// This is used for API version configuration for both frontends
+// TODO: rename system property
+internal val API_VERSION: String = System.getProperty("fir.bench.language.version", "1.4")
+
+// This is used for language version configuration for K2 only. K1 always uses default version
+private val LANGUAGE_VERSION_K2: String = System.getProperty("fir.bench.language.version.k2", "2.0")
 
 class FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
 
     override fun configureArguments(args: K2JVMCompilerArguments, moduleData: ModuleData) {
         args.useK2 = true
         args.useIR = true
-        args.languageVersion = "2.0"
-        args.apiVersion = LANGUAGE_VERSION
+        args.languageVersion = LANGUAGE_VERSION_K2
+        args.apiVersion = API_VERSION
         args.jvmDefault = "compatibility"
         args.optIn = moduleData.optInAnnotations.toTypedArray() + arrayOf(
             "kotlin.RequiresOptIn",