Add an e2e case for the parallel selector configured with maxParallel: 1.
With only one node racing per connection the dead node is no longer
masked by the live one, so the requests alternate between success and
failure - that alternation is what proves the limit is applied.
Requires go-gost/x with the parallel selector node limit (gost#916).
gost's go.mod still pins v0.19.5, so this test only goes green once
that release lands and the dependency is bumped.
ginuerzh
66ef4a3e14