Remove a race condition in `watch`.

Currently, `dir_state` can fail in unfortuate ways when a file disappears
between reading a directory listing and calculating a hash of its contents;
this patch fixes the problem by catching the failure and returning NA.
Add code to test `test_that`.

This change adds a new reporter that simply gathers expectation failures
silently, and rewrites `inst/tests/test-xxx.r` to use it.
Merge pull request #80 from craigcitro/tests

Make testthat tests more reliable when called from the interpreter.
Merge pull request #83 from craigcitro/testtest

Add code to test test_that calls.