[R-package] fixed minor issues with tests and documentation

[jameslamb]

This PR addresses a few issues found during #2530 . Splitting them out into a separate PR because I think they can go through a much quicker review, and we need to fix them regardless of what is decided in #2530 .

1. extra comma in lgb.cv.R. fixes:

lgb.cv : <anonymous>: missing arguments not allowed in calls to ‘list’

2. missing quote in documentation in lgb.prepare_rules2.R. fixes:

Error: LaTeX error. Mismatched quote or braces

3. two uses of expect_true() where test_that() should have been used in test_parameters.R
4. fix to avoid warning from R CMD check

multiple.tree.interprete: no visible binding for global variable
  ‘abs_contribution’
Undefined global functions or variables:
  abs_contribution

None of these changes impact the roxygen documentation, so there are no man/ file changes in the diff.

[StrikerRUS]

LGTM!