Update file name generation (#100)

Author: ZimboPro

Diff for: lib/src/screenshot/screenshot.dart

@@ -81,7 +81,7 @@ Future<Screenshot> takeScreenshot({
     final file = frame?.uri.pathSegments.last.replaceFirst('.dart', '');
     final line = frame?.line;
     if (file != null && line != null) {
-      return '$file:$line';
+      return '${file}_$line';
     }
     if (file != null) {
       return file;

Diff for: test/screenshot/screenshot_test.dart

@@ -178,7 +178,7 @@ void main() {
   testWidgets('screenshot file name contains test file name', (tester) async {
     final shot = await takeScreenshot();
     final lineNumber = _currentLineNumber() - 1;
-    expect(shot.file.name, contains('screenshot_test:$lineNumber'));
+    expect(shot.file.name, contains('screenshot_test_$lineNumber'));
   });
 
   testWidgets('name gets escaped to prevent slashes', (tester) async {