use a final var in the test

Author: BobTheBuidler

mypyc/test-data/irbuild-tuple.test

@@ -434,11 +434,15 @@ L4:
     return 1
 
 [case testTupleBuiltFromConstantFolding]
+from typing import Final
+
+c: Final = "c"
+
 def f2(val: str) -> str:
     return val + "f2"
 
 def test() -> None:
-    a = tuple(f2(x) for x in "ab" + "c")
+    a = tuple(f2(x) for x in "ab" + c)
 
 [out]
 def f2(val):