Compose urls in Test.serverAddress using Node's url library

[tprobinson]

When making supertest calls with URLs that do not start with a slash, all sorts of wacky errors can result due to the way the serverAddress is composed via string concatenation.

Using Node's url library will give more reliable and error-resilient results.

[coveralls]

Pull Request Test Coverage Report for Build 410

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.297%

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Totals
Change from base Build 404:	0.0%
Covered Lines:	138
Relevant Lines:	142

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💛 - Coveralls

[tateexon]

Any chance of getting this merged? Would be very nice to have.