[LL FIR] KT-50732 Add support for LL FIR-specific tests (.ll.kt)

- `.ll.kt` test data can be added in cases where LL FIR resolution
  legally diverges from K2 compiler results.
- Each `.ll.kt` test is prefixed with an `LL_FIR_DIVERGENCE` directive
  which must explain why the test may diverge from K2 compiler results.
  - `LLFirDivergenceCommentChecker` ensures that each `.ll.kt` file
    contains an `LL_FIR_DIVERGENCE` directive.
- `LLFirIdenticalChecker` results in an assertion error if the `.ll.kt`
  test and its base test are completely identical, including in their
  meta info (but ignoring `LL_FIR_DIVERGENCE`).
  - The checker additionally ensures that the base source file and the
    `.ll.kt` source file have identical Kotlin source code (ignoring
    meta info and `LL_FIR_DIVERGENCE`). This ensures that both tests
    test the exact same thing.
- `.ll.kt` files are ignored by select test generators, in addition to
  `.fir.kt` files.

generators/analysis-api-generator/tests/org/jetbrains/kotlin/generators/tests/analysis/api/firLowLevel.kt

@@ -16,9 +16,7 @@ import org.jetbrains.kotlin.generators.TestGroupSuite
 import org.jetbrains.kotlin.generators.util.TestGeneratorUtil
 import org.jetbrains.kotlin.spec.utils.GeneralConfiguration
 import org.jetbrains.kotlin.spec.utils.tasks.detectDirsWithTestsMapFileOnly
-import org.jetbrains.kotlin.test.runners.AbstractFirDiagnosticTestSpec
-
-private const val excludedFirTestdataPattern = "^(.+)\\.fir\\.kts?\$"
+import org.jetbrains.kotlin.test.utils.CUSTOM_TEST_DATA_EXTENSION_PATTERN
 
 internal fun TestGroupSuite.generateFirLowLevelApiTests() {
     testGroup("analysis/low-level-api-fir/tests", "compiler/fir/raw-fir/psi2fir/testData") {
@@ -78,11 +76,11 @@ internal fun TestGroupSuite.generateFirLowLevelApiTests() {
         testClass<AbstractDiagnosisCompilerTestDataTest>(suiteTestClassName = "DiagnosisCompilerTestFE10TestdataTestGenerated") {
             model(
                 "diagnostics/tests",
-                excludedPattern = excludedFirTestdataPattern,
+                excludedPattern = CUSTOM_TEST_DATA_EXTENSION_PATTERN,
             )
             model(
                 "diagnostics/testsWithStdLib",
-                excludedPattern = excludedFirTestdataPattern,
+                excludedPattern = CUSTOM_TEST_DATA_EXTENSION_PATTERN,
                 excludeDirs = listOf("native")
             )
         }
@@ -94,7 +92,7 @@ internal fun TestGroupSuite.generateFirLowLevelApiTests() {
             model(
                 "diagnostics",
                 excludeDirs = listOf("helpers") + detectDirsWithTestsMapFileOnly("diagnostics"),
-                excludedPattern = excludedFirTestdataPattern,
+                excludedPattern = CUSTOM_TEST_DATA_EXTENSION_PATTERN,
             )
         }
     }