Improve css_loadfile() tests
- use TMP_DIR constant rather than /tmp
- swap order of parameters in csstest() to match reported 'expected' & 'actual'
- add tests for use of 'url()' in @import
- add tests for @import of '.less' files (these test will fail per FS#2875)
add DOKU_INC to less import directories
update url/file rewriting in css_loadfile() to support @import of less files
additional tests where for relative path to @import less file
Test to ensure less can parse the @import rewritten by css_loadfile()

This test is horrible, but I believe necessary to ensure that the
@import of less files actually works.

It is horrible as its not a unit test and its not a true functional
test.  It probably implies the code in css_out() should be refactored
to make it easier to test intermediate results.
fix sp. in comment
refactor to improve elegance
Merge pull request #407 from splitbrain/FS#2875

FS#2875 : Allow less to parse @import of .less files