REGTESTS: http-errorfiles: test the errorloc directives

REGTESTS: http-errorfiles: test the errorloc directives

The errorloc, errorloc302 and errorloc303 directives had no coverage
while errorfile and http-error do. The test checks the three forms on a
503 caused by an unreachable server and on a 403 returned by a deny
rule, verifies that "errorloc" is an alias for "errorloc302", that a
relative location is accepted, and that a status code for which no
errorloc was declared still gets the default error page.