Add test that ensures `@context` value is preserved.

[BigBlueHat]

There is no test in the EdDSA test suite that ensures that the @context value is preserved.
w3c/vc-di-eddsa#102 (comment)

Awaiting updated test vectors from @Wind4Greg to be added to the spec:
w3c/vc-di-eddsa#102 (comment)