Migrates all remaining usages of
`contract.Assert*` and `contract.Require*` to the f variants,
which require adding meaningful error messages.
There were a couple cases where a `testing.T` or `testing.B`
was already available.
For those, this uses t.FailNow or require.NoError.
Refs #12132
This ensures the schema is fully validated by the spec parser, the old
schema wasn't even codegen-able because of various fields not being
linked up correctly.