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It covers AssignKind, AssignNode, and AsKind for every combination of
assembler kind and method.

We also verify that a constructed scalar node behaves the same with
AsKind when using its representation, like the old test.

There's effectively a triple loop as a test table, so the subtest name
has up to three components separated by dashes, such as:

	TestSchema/Scalars/Bytes-AssignNode-String

We also use this test as a demo of quicktest instead of go-wish.

Finally, adapt bindnode to pass these tests just like codegen. This was
mainly a bunch of TODOs in the relevant methods.