Make tests take less time

[martintoreilly]

Tests are occasionally timing out on Travis CI. Review and revise tests to reduce time taken to run.

• Travis CI doesn't seem to be caching R or the required packages. See issue Fix package caching in Travis CI #95
• The tests that run over the random graphs take a long time. Consider whether the assurance these provide can be either (i) provided by other tests or (ii) not run on every commit. See e.g. issue Flag long running tests with skip_on_cran() to skip on CRAN and Travis CI #39 (skip tests on CRAN).
• Calculating code coverage takes almost as long as running the tests. See issue Speed up code coverage processing #103).