Move LintMetrics to promtest package

[stephen-soltesz]

By importing the system "testing" package, any users of the prometheusx package inherited a long list of "testing" flags that would almost never be needed (see below) by users of prometheusx.MustStartPrometheus.

This change adds a new sub package of prometheusx named promtest that contains the LintMetrics function. No new code was added.

This change is not backward compatible.

2019/04/01 02:34:56 argsfromenv.go:38: Argument test.bench=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.benchmem=false
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.benchtime=1s
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.blockprofile=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.blockprofilerate=1
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.count=1
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.coverprofile=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.cpu=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.cpuprofile=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.failfast=false
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.list=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.memprofile=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.memprofilerate=0
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.mutexprofile=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.mutexprofilefraction=1
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.outputdir=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.parallel=4
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.run=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.short=false
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.testlogfile=/var/log/...
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.timeout=0s
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.trace=
2019/04/01 02:34:56 argsfromenv.go:38: Argument test.v=true

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 376

• 23 of 23 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.986%

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Totals
Change from base Build 374:	0.0%
Covered Lines:	406
Relevant Lines:	564

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💛 - Coveralls