Move SampleTableBoxesTest.java into third_party/.../muxer/...

Following changes are included:
1. Move test file into third_party/.../muxer/...
2. Rewrite tests for stts box and stsz box to use dump file infra instead of Mp4Slicer.
3. Remove existing test cases related to stts box which are kind of duplicate as it tries to cover the scenarios which are already covered by "DurationsVuForStts" related test cases.

PiperOrigin-RevId: 510977119

testdata/src/test/assets/muxerdumps/stsz_box.dump

+stsz (36 bytes):
+  Data = length 28, hash 2257E30F

testdata/src/test/assets/muxerdumps/stts_box_all_different_sample_durations.dump

+stts (48 bytes):
+  Data = length 40, hash 558A08D9

testdata/src/test/assets/muxerdumps/stts_box_few_same_sample_durations.dump

+stts (32 bytes):
+  Data = length 24, hash AA341F5

testdata/src/test/assets/muxerdumps/stts_box_single_sample_duration.dump

+stts (24 bytes):
+  Data = length 16, hash E4FC6496