Add release-mode test runs for macOS and Linux

[natecook1000]

This adds a Build / Test [release] step to the tests for macOS and Linux, so that tests can be run in both modes. It includes new inputs for:

• macos_release_build_command (default: xcrun swift test -c release)
• linux_release_build_command (default: swift test -c release)
• enable_release_tests (default: false)

[rauhul]

Why not just run in release always?

[FranzBusch]

Why not just run in release always?

Most packages don't support building tests in release mode due to @testable imports AFAIK

[rauhul]

@testable supports release mode. It just makes internal api visible outside the module unit.

[natecook1000]

Huh, that isn't my experience! I thought it would just be easier to not block on this for packages that couldn't support it fully.

[ahoppen]

I think you need to pass --enable-testable-imports to test a package with @testable imports in release mode.

[jrflat]

FWIW even with --enable-testable-imports, System was still failing to build tests in release mode (due to some #if DEBUG conditionals, fixed by PR above). We would probably need to keep enable_release_tests defaulting to false or otherwise track down/help fix potential failures in projects using this workflow.

[MahdiBM]

The way to enable @testable imports even in release builds is to use -Xswiftc -enable-testing like I've added a few months ago to Vapor's reusable CI for release-mode testing of packages when possible.
I'm not quite sure why --enable-testable-imports doesn't cut it. Could be a bug.