Use `Uri.toFilePath()` instead of `Uri.path` for locating tests (dart-lang/markdown#396)

Otherwise the paths are wrongly formatted in windows resulting in test failures.

pkgs/markdown/test/util.dart

diff --git a/pkgs/markdown/test/util.dart b/pkgs/markdown/test/util.dart
index 362649b..86e78c8 100644
--- a/pkgs/markdown/test/util.dart
+++ b/pkgs/markdown/test/util.dart
@@ -27,7 +27,7 @@
 Future<String> get markdownPackageRoot async =>
     p.dirname(p.dirname((await Isolate.resolvePackageUri(
             Uri.parse('package:markdown/markdown.dart')))!
-        .path));
+        .toFilePath()));
 
 void testFile(
   String file, {

pkgs/markdown/tool/expected_output.dart

diff --git a/pkgs/markdown/tool/expected_output.dart b/pkgs/markdown/tool/expected_output.dart
index 8561c8e..3d3b7e4 100644
--- a/pkgs/markdown/tool/expected_output.dart
+++ b/pkgs/markdown/tool/expected_output.dart
@@ -119,7 +119,7 @@
 }) async* {
   var markdownLibRoot = p.dirname((await Isolate.resolvePackageUri(
           Uri.parse('package:markdown/markdown.dart')))!
-      .path);
+      .toFilePath());
   var directory =
       p.joinAll([p.dirname(markdownLibRoot), 'test', testDirectory]);
   for (var dataCase in _dataCases(