e2ee: add unit tests

[fippo]

Both for just encode function as well as an end-to-end test.

[saghul]

❤️

[fippo]

   SyntaxError: /home/runner/work/lib-jitsi-meet/lib-jitsi-meet/modules/e2ee/Context.js: Identifier directly after number (60:27)
    58 |         // Must be incremented on every frame that is sent, can be reset on
    59 |         // key changes.
  > 60 |         this._sendCount = 0n;

That is an odd failure. This line isn't even in the change? Updated to use BigInt instead

[fippo]

there is a test for this but there should be a test for "it errors when running out of window" :-)