Flow analysis: Fix promote()'s update of tested set. (#1116)

When promoting via a test on type T, the type that should be added to
the test set is T, even if the actual promoted type is different.
This brings the spec in line with the implementation.

Author: stereotype441

resources/type-system/flow-analysis.md

@@ -443,7 +443,7 @@ Definitions:
       - Else if `S` is `X extends R` then let `T1` = `X & T`
       - Else If `S` is `X & R` then let `T1` = `X & T`
       - Else `x` is not promotable (shouldn't happen since we checked above)
-      - Let `VM2 = VariableModel(declared, T1::promoted, T1::tested, assigned,
+      - Let `VM2 = VariableModel(declared, T1::promoted, T::tested, assigned,
       unassigned, captured)`
       - Let `M2 = FlowModel(r, VI[x -> VM2])`
       - If `T1 <: Never` then `M3` = `unreachable(M2)`, otherwise `M3` = `M2`