R: rewrote test cases using the testthat package.

Conflicts:
	interfaces/R/tests/bug-1073705-indexing.Rout.save
	interfaces/R/tests/get.shortest.paths.R
	interfaces/R/tests/get.shortest.paths.Rout.save
	interfaces/R/tests/leading.eigenvector.community.R
	interfaces/R/tests/leading.eigenvector.community.Rout.save
	interfaces/R/tests/modularity.matrix.Rout.save
	interfaces/R/tests/pajek.Rout.save
R: put tests in the package, and provide a function to run them. Closes issue #485.