validate: Add playbin3 suburi test

This test should help ensuring this issue no longer happens:
gstreamer/gst-plugins-base!385

medias/defaults/subtitles/tick.srt

+0
+00:00:00,000 --> 00:00:00,500
+00:00:00
+
+1
+00:00:01,000 --> 00:00:01,500
+00:00:01
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+2
+00:00:02,000 --> 00:00:02,500
+00:00:02
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+3
+00:00:03,000 --> 00:00:03,500
+00:00:03
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+4
+00:00:04,000 --> 00:00:04,500
+00:00:04
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+5
+00:00:05,000 --> 00:00:05,500
+00:00:05
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+6
+00:00:06,000 --> 00:00:06,500
+00:00:06
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+7
+00:00:07,000 --> 00:00:07,500
+00:00:07
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+8
+00:00:08,000 --> 00:00:08,500
+00:00:08
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+9
+00:00:09,000 --> 00:00:09,500
+00:00:09
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+10
+00:00:10,000 --> 00:00:10,500
+00:00:10