testkit: give up on waiting for the RPC server to shutdown after 1s

[dirkmc]

Some itests fail to complete because the testkit RPC server blocks waiting for all incoming connections to be closed.
This PR changes the shutdown code to wait up to 1s and then give up on shutting down the server.

[codecov]

Codecov Report

Merging #8450 (c34ade9) into master (f652dd3) will decrease coverage by 0.00%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master    #8450      +/-   ##
==========================================
- Coverage   40.67%   40.66%   -0.01%     
==========================================
  Files         686      686              
  Lines       75426    75439      +13     
==========================================
- Hits        30678    30677       -1     
- Misses      39450    39464      +14     
  Partials     5298     5298              

Impacted Files	Coverage Δ
itests/kit/rpc.go	95.00% <78.57%> (-5.00%)	⬇️
markets/loggers/loggers.go	89.28% <0.00%> (-10.72%)	⬇️
extern/sector-storage/worker_tracked.go	79.82% <0.00%> (-7.02%)	⬇️
chain/stmgr/tracers.go	84.21% <0.00%> (-5.27%)	⬇️
extern/storage-sealing/currentdealinfo.go	72.97% <0.00%> (-3.61%)	⬇️
chain/sub/incoming.go	45.25% <0.00%> (-2.45%)	⬇️
blockstore/autobatch.go	56.30% <0.00%> (-1.69%)	⬇️
chain/store/store.go	64.33% <0.00%> (-1.17%)	⬇️
chain/sync.go	70.52% <0.00%> (-0.35%)	⬇️
chain/messagepool/messagepool.go	63.65% <0.00%> (-0.25%)	⬇️
... and 7 more