internal/jsonrpc2/servertest: replace closerList with connList
internal/jsonrpc2/servertest: replace closerList with connList

closerList was already unnecessarily abstract, and tracking connections
will allow us to also wait for all connections to finish.

Share functionality by embedding this type in PipeServer, TCPServer.

Change-Id: Ib2cb2157c1477f904bc278aa91902f5c633afe13
Reviewed-on: https://go-review.googlesource.com/c/tools/+/238547
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools