Move some tests to `TestNetworkTransport`

[martinbonnin]

Dogfood TestNetworkTransport in our own tests.

[martinbonnin]

Decide and document when to use:

• SampleServer (with Ktor): HTTP2/TLS/programmatic response. Ktor has no JS
• MockServer: JS/Native/raw socket access
• TestNetworkTransport: No network, faster(?)