SampleBuilder: add memory leak test

[at-wat]

Test that the input RTP packets are unreferenced after all samples are popped.

On v4.0.0-beta.13

=== RUN   TestSampleBuilderPacketUnreference
--- PASS: TestSampleBuilderPacketUnreference (0.05s)

On v4.0.0-beta.14

=== RUN   TestSampleBuilderPacketUnreference
    samplebuilder_test.go:570: 
        	Error Trace:	/home/at-wat/go/src/github.com/pion/webrtc/pkg/media/samplebuilder/samplebuilder_test.go:570
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 65536
        	Test:       	TestSampleBuilderPacketUnreference
--- FAIL: TestSampleBuilderPacketUnreference (0.08s)

Reference issue

Reproduce the memory leak reported in #2778

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.97%. Comparing base (d9994bd) to head (1528cc6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2781      +/-   ##
==========================================
+ Coverage   78.88%   78.97%   +0.08%     
==========================================
  Files          89       89              
  Lines        8432     8432              
==========================================
+ Hits         6652     6659       +7     
+ Misses       1294     1290       -4     
+ Partials      486      483       -3     

Flag	Coverage Δ
go	80.54% <ø> (+0.09%)	⬆️
wasm	65.11% <ø> (ø)

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