[R-package] Added unit tests on creating Booster from file or string

[jameslamb]

This PR adds unit tests on the code in the R package that handles saving a Booster to file or string and creating a new Booster from a file or string. This brings the unit test coverage on the package from 78.20% to 79.71%.

Importantly, it covers calls from R to C++ functions LGBM_BoosterSaveModelToString_R, LGBM_BoosterCreateFromModelfile_R, and LGBM_BoosterLoadModelFromString_R which were previously not covered (see #2944 for why this is valuable).

This PR does not change the behavior of lgb.load() at all, but it does reorganize the internals to cut out some redundant checks and make the logic a little easier to follow.

This PR also removes an unnecessary step from the docs on generating test coverage in R-package/README.md. Removing the build/ directory is no longer necessary, as of #2909 .

[jameslamb]

Rebased to master, to get the changes from #2954 , so the R test issues should be resolved. That will also fix the weird state of the Travis build, which was a result of the recent Travis outage.

[jameslamb]

gently ping @Laurae2 @guolinke for a review