fix tests warnings for CRAN submission

[jayhesselberth]

After #328 there are a few warnings thrown when the tests are run locally. These will need to be fixed before a CRAN submission.

test_merge.r:178: warning: Test stranded merge with bedPlus files that have strand
2 parsing failures.
row # A tibble: 2 x 5 col     row col       expected   actual file                                        expected   <int> <chr>     <chr>      <chr>  <chr>                                       actual 1     1 cds_start an integer foo    '/Users/jayhesselberth/devel/valr/inst/ext… file 2     2 cds_start an integer bar    '/Users/jayhesselberth/devel/valr/inst/ext…

test_slop.r:117: warning: test that old floating-point issues are solved
Column `chrom` joining character vector and factor, coercing into character vector

test_slop.r:129: warning: test that old floating-point issues are solved
Column `chrom` joining character vector and factor, coercing into character vector

test_slop.r:188: warning: test crossover during negative slop resulting in 0 length intervals are tossed out
1 intervals generated with same start and end were discarded

[jayhesselberth]

You may have to use testthat::expect_warning() for some of these. I.e., test for the warning explicitly, but don't let the test return a warning.