Rename test files to work better with usethis. (align them with R files)

[olivroy]

this way, it is easier to jump between R/ and tests/ with usethis::use_r(), and usethis::use_test().

s2 was not recognized as test because it didn't have the 'test' prefix

If merged, this would create conflicts with #2059, but I wouldn't mind solving them if you'd like me