Fix minor typo in check_package documentation

Documentation on line 5 in `testthat/R/test-package.r` has 

> either \code{inst/tests}, or (better) \code{tests/testhat}.

Latter directory name should probably be `\code{tests/testthat}`.
removed the warning in case the srcref of the expect_call can not be found, e.g. for a bare expectation, as reported by Hadley
Merge pull request #96 from kforner/srcref_fix

removed the warning in case the srcref of the expect_call can not be fou...
Support relative paths in source_dir
Simplify relative path logic
Merge pull request #99 from jmcphers/master

Support relative paths in source_dir
Merge pull request #88 from jlehtoma/master

Fix minor typo in check_package documentation
merged from testthat hadley master