Don't warn for missing appname if version is NULL

* if version is NULL we don't need to be warned that it will be ignored
* this was a hurried addition in 8428ce0301 on top of Gabor Csardi's
  a4ef238573
Switch tests to tests/testthat

* this is the new recommended location, allowing tests to be installed or not
* see https://github.com/hadley/testthat#integration-with-r-cmd-check
Fix tests on Windows by wrapping baseline paths in file_path

* use internal file_path function to normalise baseline paths in the same
  way as the xxx_dir() functions
* using file_path avoids having to use:
  normalizePath(file.path(), mustWork=FALSE)
Merge branch 'fix/appname-warning' into dev

* fix/appname-warning:
  Don't warn for missing appname if version is NULL
Merge branch 'fix/wintests' into dev

* fix/wintests:
  Fix tests on Windows by wrapping baseline paths in file_path
  Switch tests to tests/testthat

Conflicts:
	tests/testthat/test-site_data_dir.r
	tests/testthat/test-user_cache_dir.r
	tests/testthat/test-user_config_dir.r
	tests/testthat/test-user_log_dir.r

(whitespace)