test: add a context manager for error injection to alternator
test: add a context manager for error injection to alternator

With the new context manager it's now easier to request an error
to be injected via REST API. Note that error injection is only
enabled in certain build modes (dev, debug, sanitize)
and the test case will be skipped if it's not possible to use
this mechanism.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla