The e2e tests can be flaky due to various tests mutating shared mutable
fixtures, causing non-deterministic behavior depending on the test set, order,
etc.
For example, `configTLS` is mutated in at least two tests in such a way that the
config is potentially invalidated for any subsequent test running in the same
process (e.g. by setting the `enableV2` field). This particular example caused
a substantial amount of confusion diagnosing the new test introduced for
https://github.com/etcd-io/etcd/pull/12370.
Independent tests should not share mutable state unless deliberately. This patch
refactors the e2e test config fixtures to safeguard against these problems by
replacing the package variables (which cannot easily be made immutable) with
functions that return new instances.
