when(condition:) to switch over building for running vs testing

[rauhul]

Description

I'd like to conditionally define a flag when building a test to allow me to interpose an implementation detail of my library from a test.

I think this would look like:

.target(
  name: "Foo",
  cSettings: [
    .define("TESTING", condition: .when(buildKind: .test)),
  ]),

Currently one has to use environment variable trickery to enable this sort of behavior which is very gross

[rauhul]

Example code that could be replaced by this feature:

// FIXME: https://github.com/apple/swift-package-manager/issues/6992
// Replace this with .define("TESTING", condition: .when(buildKind: .testing))
import Foundation

// Modify this variable to avoid reopening Xcode when changing modes.
let overrideFooEnableFooTests: Bool? = true
let envFooEnableFooTests = ProcessInfo
  .processInfo.environment["FOO_ENABLE_FOO_TESTS"] != nil
if overrideFooEnableFooTests ?? envFooEnableFooTests {
  struct EnableFooTestsError: Error, CustomStringConvertible {
    var description = "Failed to locate target 'FooSupport'"
  }

  let fooSupportIndex = package
    .targets
    .firstIndex { $0.name == "FooSupport" }
  guard let fooSupportIndex = fooSupportIndex else {
    throw EnableFooTestsError()
  }
  var fooSupportCSettings = package.targets[fooSupportIndex].cSettings ?? []
  fooSupportCSettings.append(.define("TESTING"))
  package.targets[fooSupportIndex].cSettings = fooSupportCSettings

  package.targets.append(
    .testTarget(
      name: "FooTests",
      dependencies: ["Foo"]))
}