squid-conf-tests should test installed Squid (#1204)

squid-conf-tests should test installed Squid (#1204)

    FATAL: ..._inst/etc/mime.conf: (2) No such file or directory
    FATAL: ..._inst/share/icons: (2) No such file or directory

Our squid-conf-tests are running "squid -k parse" that requires icons
and other supplementary files to be _installed_. Thus, squid-conf-tests
must run during "make installcheck" rather than during "make check".

With this change, "make distcheck" triggers squid-conf-tests at the
right time, after "all", "check", and "install" targets are built.
Also, we are now testing installed Squid binaries, as we should.