Fix: Silence SwiftPM unhandled-file warning in StringZillaTests

Both `StringZilla` and `StringZillaTests` use `path: "swift"`. The
former excludes `Test.swift`; the latter only set `sources:` but did
not exclude the sibling source, so SwiftPM emitted

  warning: 'stringzilla': found 1 file(s) which are unhandled;
  explicitly declare them as resources or exclude from the target
      .../swift/StringProtocol+StringZilla.swift

on every macOS Pre-Release run. Add the symmetric `exclude:` entry to
the test target.

Author: ashvardanian

Package.swift

@@ -42,6 +42,7 @@ let package = Package(
             name: "StringZillaTests",
             dependencies: ["StringZilla"],
             path: "swift",
+            exclude: ["StringProtocol+StringZilla.swift"],
             sources: ["Test.swift"]
         ),
     ],