On a decently fast system, some files may be fetched, modified and then
fetched again within the same second. In that case, the web server
replies with a code 304 ("Not modified") to the 2nd query, causing some
tests to fail.
This commit forces the web server to ignore `If-Modified-Since` HTTP
headers, effectively disabling caching in order to mitigate the problem.
Rather than this weird shell loop with a timeout and polling we
move the httpd spawning entirely into the python code, and use
a pipe to synchronize the spawning. This way we can also use
the shell job control to properly clean up any running processes
from the test suite.
Additionally, this adds some (lame) support for token handling in the
test webserver, where you for any file foo can create a foo.need_token
containing a token that is needed for that file.
