Rtex code extraction problematic

[AshesITR]

When bringing #165 up to speed, I noticed a test failure for Rtex, caused by the behavior of get_source_expressions():

get_source_expressions("tests/testthat/knitr_formats/test.Rtex")$lines[[25]]
#> [1] " "

The original file line in question is "%", where the % is masked by whitespace as part of the Rtex markup.
This results in a false positive trailing_whitespace_linter() hit for that line once it's extended to cover all lines and not just lines with expressions.

[MichaelChirico]

Copying over some more context from a TODO comment:

file_lines contains a whitespace on the final line for Rtex, because that is used to mark the Rtex escape char "%" as well. c.f. get_source_expressions("tests/testthat/knitr_formats/test.Rtex")$lines[[25]]