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%.

[jmschonfeld]

@marcprux the copy of XCTest source code in this repo is not the official source for XCTest on non-Darwin platforms. It's an older copy of the source code solely used for the Foundation project to avoid layering issues (due to the XCTest dependency on Foundation). Is the issue you're seeing specific to this Foundation project (I don't think we have performance testing in here so I suspect not)? If not, you likely want to investigate this with the XCTest team in their repo at https://github.com/swiftlang/swift-corelibs-xctest

[marcprux]

Oops, you are right! I've re-filed at swiftlang/swift-corelibs-xctest#506 and closed this one.