test: normalize expected paths for Windows

When testing on Windows, the tool currently prints paths with a `/` path
separator rather than the canonical `\` separator for the platform.  The
resulting path is valid and usable.  However, this causes match failures
due to the difference in the path separator.  The `slash` module permits
us to normalize the path for the tests.  This should have no impact when
running on Unix platforms, but allows greater portability to Windows.

test/linters/params/swift-format-official.js

@@ -1,4 +1,5 @@
 const { join } = require("path");
+const { slash } = require("slash");
 
 const SwiftFormatOfficial = require("../../../src/linters/swift-format-official");
 
@@ -8,10 +9,10 @@ const commandPrefix = "";
 const extensions = ["swift"];
 
 function getLintParams(dir) {
-	const warning1 = `${join(dir, "file2.swift")}:2:22: warning: [DoNotUseSemicolons]: remove ';'`;
-	const warning2 = `${join(dir, "file1.swift")}:3:35: warning: [RemoveLine]: remove line break`;
-	const warning3 = `${join(dir, "file1.swift")}:7:1: warning: [Indentation] replace leading whitespace with 2 spaces`;
-	const warning4 = `${join(dir, "file1.swift")}:7:23: warning: [Spacing]: add 1 space`;
+	const warning1 = `${slash(join(dir, "file2.swift"))}:2:22: warning: [DoNotUseSemicolons]: remove ';'`;
+	const warning2 = `${slash(join(dir, "file1.swift"))}:3:35: warning: [RemoveLine]: remove line break`;
+	const warning3 = `${slash(join(dir, "file1.swift"))}:7:1: warning: [Indentation] replace leading whitespace with 2 spaces`;
+	const warning4 = `${slash(join(dir, "file1.swift"))}:7:23: warning: [Spacing]: add 1 space`;
 	// Files on macOS are not sorted.
 	const stderr =
 		process.platform === "darwin"