fix(verifier): acquire HTTP port atomically

[alexeits]

Acquire the port in VerifyMessageProvider and VerifyProvider atomically by using the listener used for a free port acquisition for the HTTP server.

This prevents a race condition when multiple pact verifiers run in parallel and compete for free ports, which could result in errors like

Expected server to start < 10s. Timed out waiting for http verification proxy on port 34425 - check for errors

[coveralls]

Pull Request Test Coverage Report for Build 6305748030

• 20 of 28 (71.43%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 77.392%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
proxy/http.go	7	10	70.0%
dsl/pact.go	13	18	72.22%

Files with Coverage Reduction	New Missed Lines	%
proxy/http.go	1	36.04%
dsl/pact.go	3	83.53%

Totals
Change from base Build 6305094254:	-0.1%
Covered Lines:	1472
Relevant Lines:	1902

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💛 - Coveralls