Consider renaming SWIFT_PM_TEST_LIBRARY to a more general name like APPLE_TEST_LIBRARY

[jakepetroules]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI.

Description

This env var is convenient for shared libraries which want to have support code that might be run by either an XCTest or SwiftTesting test case.

However, I suggest we name this more generally (for example, APPLE_TEST_LIBRARY) because it's not SwifPM-specific, and other build tools / IDEs might want to have a uniform way to identify the Apple-provided testing library in use.

I also expect we wouldn't set this environment variable if SwiftPM gained direct support for additional testing libraries from other platforms/environments, since those would likely be run via their own test harnesses which might define their own environment variables across different IDE integrations.

Expected behavior

No response

Actual behavior

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Steps to reproduce

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Swift Package Manager version/commit hash

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Swift & OS version (output of swift --version ; uname -a)

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[grynspan]

This flag is meant to be used by SwiftPM and is not related to Apple specifically.