Make function tests a little less brittle.

Function tests depended on a global counter deep in constant folding
logic being just the right value. The value depends on exactly what
other tests involving constant folding run in the same process.

This fix adds a function that constant folds a small graph and
parses the counter value from resulting graph node name. The tests
can they base their expectations off this value. This is still not ideal
as multiple tests can theoretically run in parallel, but better than
before.

PiperOrigin-RevId: 218898255