Check for environment XCTEST_MEASURE_MAX_STDDEV for maxRelativeStandardDeviation

[marcprux]

The XCTest.measure currently uses 10.0 as the hardwired maxRelativeStandardDeviation, which can lead to test failures where a test is expected to have a high standard deviation:

PerformanceTests.swift:90: Test Case 'PerformanceTests.testUsingLoadWithUTF16' measured [Time, seconds] average: 0.884, relative standard deviation: 47.026%, values: [1.063468, 1.418311, 1.400721, 1.412482, 0.927719, 0.639287, 0.497485, 0.438305, 0.601177, 0.436784], performanceMetricID:org.swift.XCTPerformanceMetric_WallClockTime, maxPercentRelativeStandardDeviation: 10.000%, maxStandardDeviation: 0.100
PerformanceTests.swift:90: error: PerformanceTests.testUsingLoadWithUTF16 : failed: The relative standard deviation of the measurements is 47.026% which is higher than the max allowed of 10.000%.

This PR adds a check for the environment variable XCTEST_MEASURE_MAX_STDDEV, which it will use as the default maximum if it is set. Otherwise, it will fall back to 10%.

[marcprux]

This can be worked around with a technique like jpsim/Yams@67c6444

[grynspan]

@briancroom @stmontgomery Does Apple's XCTest have some equivalent mechanism?