Set subtitle track to default in sample_with_srt.mkv

This will mean the track is automatically used when running in a
playback test.

Command used to change the flag:
```
$ mkvpropedit sample_with_srt.mkv --edit track:3 --set flag-default=1
```

PiperOrigin-RevId: 345436666

testdata/src/test/assets/extractordumps/mkv/sample_with_srt.mkv.0.dump

@@ -271,6 +271,7 @@ track 3:
   format 0:
     id = 3
     sampleMimeType = application/x-subrip
+    selectionFlags = 1
     language = en
     label = Subs Label
   sample 0:

testdata/src/test/assets/extractordumps/mkv/sample_with_srt.mkv.1.dump

@@ -271,6 +271,7 @@ track 3:
   format 0:
     id = 3
     sampleMimeType = application/x-subrip
+    selectionFlags = 1
     language = en
     label = Subs Label
   sample 0:

testdata/src/test/assets/extractordumps/mkv/sample_with_srt.mkv.2.dump

@@ -271,6 +271,7 @@ track 3:
   format 0:
     id = 3
     sampleMimeType = application/x-subrip
+    selectionFlags = 1
     language = en
     label = Subs Label
   sample 0:

testdata/src/test/assets/extractordumps/mkv/sample_with_srt.mkv.3.dump

@@ -271,6 +271,7 @@ track 3:
   format 0:
     id = 3
     sampleMimeType = application/x-subrip
+    selectionFlags = 1
     language = en
     label = Subs Label
   sample 0:

testdata/src/test/assets/extractordumps/mkv/sample_with_srt.mkv.unknown_length.dump

@@ -271,6 +271,7 @@ track 3:
   format 0:
     id = 3
     sampleMimeType = application/x-subrip
+    selectionFlags = 1
     language = en
     label = Subs Label
   sample 0: