Improve the record layer fuzz tests

[philipch07]

Description

1. Moves the record layer fuzz tests from their own file to the test file so we don't have an extra file.
2. Also rewrites some of the record layer tests to test DTLS v1.2 instead of DTLS v1.0 since we don't support v1.0.
3. Adds new fuzz tests that checks for more than just panicking.

Reference issue

N/A, part of the effort to add fuzz tests to various Pion projects.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.06%. Comparing base (daa0fd4) to head (b2525af).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #750      +/-   ##
==========================================
- Coverage   79.20%   79.06%   -0.14%     
==========================================
  Files         101      102       +1     
  Lines        6915     6917       +2     
==========================================
- Hits         5477     5469       -8     
- Misses       1063     1070       +7     
- Partials      375      378       +3     

Flag	Coverage Δ
go	79.08% <ø> (-0.12%)	⬇️
wasm	59.89% <ø> (?)

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[JoTurk]

Thank you, and sorry for taking time to get back to you on this one

<3