peer: allow external testing of peer.Peer

[wpaulino]

The previous use of allowSelfConns prevented this, as users aren't able to invoke peer.TstAllowSelfConns themselves due to being part of a test file, which aren't exported at the library level, leading to a "disconnecting peer connected to self" error upon establishing a mock connection between two peers. By including the option at the config level instead (false by default, prevents connections to self) we enable users of the peer library to properly test the behavior of the peer.Peer struct externally.

[coveralls]

Pull Request Test Coverage Report for Build 644844459

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 52.863%

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Files with Coverage Reduction	New Missed Lines	%
integration/rpctest/utils.go	2	77.65%

Totals
Change from base Build 636275635:	0.02%
Covered Lines:	20929
Relevant Lines:	39591

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💛 - Coveralls

[Roasbeef]

LGTM 🚞