This replaces embedding of JavaScript code into the mock driver that
runs inside the cluster with Go callbacks which run inside the
e2e.test suite itself. In contrast to the JavaScript hooks, they have
direct access to all parameters and can fabricate arbitrary responses,
not just error codes.
Because the callbacks run in the same process as the test itself, it
is possible to set up two-way communication via shared variables or
channels. This opens the door for writing better tests. Some of the
existing tests that poll mock driver output could be simplified, but
that can be addressed later.
For now, only tests using hooks use embedding. How gRPC calls are
retrieved is abstracted behind the CSIMockTestDriver interface, so
tests don't need to be modified when switching between embedding
and remote mock driver.
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