Add solc 0.6.x integration tests

[cgewecke]

These tests don't pass because solidity-parser-antlr crashes on some of the new language features.

Part of #457

[codecov-io]

Codecov Report

Merging #461 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #461   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          20       20           
  Lines         779      779           
=======================================
  Hits          773      773           
  Misses          6        6

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