mrc-257: update tests to work with changes to R 3.6.0 and testthat

[richfitz]

A long sad story:

• R 3.6.0 now checks that paths are not hard coded in a package installation to support staged installs blog post which started throwing errors in the package code because of how the IR is loaded from disk. This has been changed to load-on-demand for now (this is the only substantive change to the package)
• testing this requires 3.6.0 which is done on travis but not debuggable so I am doing that with docker - so we add support for docker there
• we'd have picked this up faster if we tested against different R versions automatically so I have expanded the travis testing
• the actual test failures are very difficult to find through a change in testthat's behaviour so 27a7601 patches a lot of tests to keep it quiet

[codecov]

Codecov Report

Merging #170 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #170   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          38     38           
  Lines        4510   4504    -6     
=====================================
- Hits         4510   4504    -6

Impacted Files	Coverage Δ
R/utils.R	100% <ø> (ø)	⬆️
R/interface.R	100% <100%> (ø)	⬆️
R/generate_c_class.R	100% <100%> (ø)	⬆️

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[weshinsley]

All looks good - that's indeed a lot of updated tests...