`swift test` should warn when `--filter` excludes all tests (_à la_ XCTest.)

[grynspan]

When using XCTest and swift test --filter, if all tests are filtered out a diagnostic of the form:

warning: No matching test cases were run

Is emitted by SwiftPM. We should have a similar warning we emit from swift-testing in this scenario.

Checklist:

• [ ] Code and documentation should follow the style of the Style Guide.
• [ ] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Draft for now because we may end up needing to implement the relevant logic in a single location in SwiftPM so that the message reflects the overall output, not per-library output. TBD.

[grynspan]

No longer required due to #536 and https://github.com/swiftlang/swift-package-manager/pull/7777.