t/gitweb-lib.sh: Add support for GITWEB_TEST_INSTALLED

t/gitweb-lib.sh: Add support for GITWEB_TEST_INSTALLED

You can set the GITWEB_TEST_INSTALLED environment variable to the
gitwebdir (the directory where gitweb is installed / deployed to) of
an existing gitweb instalation, or to the pathname of installed gitweb
script, to test that installation.

This change is intended to make it possible to test that process of
installing gitweb and the modules it depends on works correctly (after
splitting gitweb).

If GITWEB_TEST_INSTALLED is used, print what script are we testing
to make it easy to spot that we test installed gitweb.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>