Increase more test timeouts, misc test tweaks

[mway]

• Increases test timeout from 5m -> 10m due to occasionally observing test runs just north of 5min
• ~Increases various other timeouts similar to #869~ reverted
• Changes relaytest.MockStats to only report stat deltas when there are stats discrepancies
• ~Adds testutils.TestServer.ChannelOpts to provide access to test server options~ moved to #871

[codecov[bot]]

Codecov Report

Merging #874 (59c9c11) into dev (f45abb5) will increase coverage by 0.20%. The diff coverage is n/a.

:exclamation: Current head 59c9c11 differs from pull request most recent head 5254b4e. Consider uploading reports for the commit 5254b4e to get more accurate results

@@            Coverage Diff             @@
##              dev     #874      +/-   ##
==========================================
+ Coverage   88.80%   89.00%   +0.20%     
==========================================
  Files          43       43              
  Lines        4439     4439              
==========================================
+ Hits         3942     3951       +9     
+ Misses        378      368      -10     
- Partials      119      120       +1     

Impacted Files	Coverage Δ
relay.go	85.68% <0.00%> (-1.71%)	:arrow_down:
inbound.go	80.82% <0.00%> (-1.56%)	:arrow_down:
outbound.go	87.13% <0.00%> (-1.17%)	:arrow_down:
root_peer_list.go	96.00% <0.00%> (+4.00%)	:arrow_up:
connection.go	89.37% <0.00%> (+4.61%)	:arrow_up:

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[cinchurge]

Thanks for making this change, the 10m test timeout definitely makes sense! The individual timeout tweaks though don't seem really necessary since we can use TEST_TIMEOUT_SCALE to adjust them at run time. Can you try it and see if it eliminates the need to adjust the timeouts?