[Spec tests] Add test for Coercion to Unit error diagnostics absence

compiler/tests-spec/testData/diagnostics/notLinked/coercion-to-unit/neg/1.fir.kt

@@ -0,0 +1,11 @@
+// TESTCASE NUMBER: 1
+
+val y0 = when (2) {
+    else -> if (true) {""}
+}
+
+val w:Any = TODO()
+
+val y1 = when (2) {
+    else -> if (true) {""} // false ok with coercion to Unit
+}

compiler/tests-spec/testData/diagnostics/notLinked/coercion-to-unit/neg/1.kt

@@ -0,0 +1,20 @@
+/*
+ * KOTLIN DIAGNOSTICS NOT LINKED SPEC TEST (NEGATIVE)
+ *
+ * SECTIONS: coercion-to-unit
+ * NUMBER: 1
+ * DESCRIPTION: Coercion to Unit error diagnostics absence
+ * ISSUES: KT-38490
+ */
+
+// TESTCASE NUMBER: 1
+
+val y0 = when (<!UNUSED_EXPRESSION!>2<!>) {
+    else -> <!INVALID_IF_AS_EXPRESSION!>if<!> (true) {""}
+}
+
+val w:Any = TODO()
+
+val y1 = when (2) {
+    else -> if (true) {""} // false ok with coercion to Unit
+}

compiler/tests-spec/testData/diagnostics/notLinked/coercion-to-unit/neg/1.txt

@@ -0,0 +1,5 @@
+package
+
+public val w: kotlin.Any
+public val y0: kotlin.Unit
+public val y1: kotlin.Unit