test: refactor pcap test to use version from rtshark

[lrstewart]

Description of changes:

I didn't like that we had to call tshark directly in the pcap test build script, so I moved that logic into rtshark: CrabeDeFrance/rtshark@1e9b7eb Now we can just get the version from rtshark.

Testing:

The test correctly still can't run on Ubuntu18 😔

error: failed to run custom build command for `pcap v0.1.0 (/home/ubuntu/s2n-tls/tests/pcap)`

Caused by:
  process didn't exit successfully: `/home/ubuntu/s2n-tls/tests/pcap/target/debug/build/pcap-a2228bd6ce729e8c/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at build.rs:116:5:
  tshark 4.2.0 required. tshark 3.6.2 found.

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