[FIR] Update/add extended checker tests

compiler/fir/analysis-tests/testData/extendedCheckers/unused/manyLocalVariables.kt

@@ -0,0 +1,19 @@
+fun foo() {
+    <!CAN_BE_VAL!>var<!> a = 1
+    var b = <!VARIABLE_INITIALIZER_IS_REDUNDANT!>2<!>
+    var c = 3
+
+    for (i in 0..5) {
+        if (a == 2) {
+            <!ASSIGNED_VALUE_IS_NEVER_READ!>b<!> = c
+            c = a
+        } else {
+            b = a
+            c = b
+        }
+    }
+
+    if (c == a) {
+        foo()
+    }
+}