gitweb: Minimal testing of gitweb caching

gitweb: Minimal testing of gitweb caching

Add basic tests of caching support to t9500-gitweb-standalone-no-errors
test: set $caching_enabled to true and check for errors for first time
run (generating cache) and second time run (retrieving from cache) for a
single view - summary view for a project.  Check also that request for
non-existent object (which results in die_error() codepath to be called)
doesn't produce errors.

Check in t9501-gitweb-standalone-http-status that request for
non-existent object produces correct output (HTTP headers and HTML
output) also when caching is enabled.

Check in the t9502-gitweb-standalone-parse-output test that gitweb
produces the same output with and without caching, for first and
second run, with binary (raw) or plain text (utf8) output.

The common routine that enables cache, gitweb_enable_caching, is
defined in t/gitweb-lib.sh

Signed-off-by: Jakub Narebski <jnareb@gmail.com>