Add testcase for invalid if statement (#15313)

tests/parser/tinvifstmt.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "invalid indentation"
+  line: 9
+  column: 3
+"""
+
+if true:
+    echo "a"
+  elif:
+    echo "b"
+ else:
+   echo "c"