make sure ftl.Map handles changes in underlying values correctly

[matt2e]

From slack:

aat
Something else re. testing
@mtoohey

• ftl.Map will need to change such that its caching is contingent on the underlying value being the same, rather than blindly doing it

ctx := ftltest.Context(
    ftltest.WithConfig(example, "hello"),
)

//Call a verb that uses ftl.Map with example config
resp, err := Verb(ctx, req)

ctx = ftltest.Context(
    ftltest.WithConfig(example, "world"),
)

//When this execution of the verb is done, it should execute again because the underlying value is different (hello -> world)
resp, err = Verb(ctx, req)