skip tests if we have no supported downloader

as seen in CPAN reports:
<<<
...retrieving failed: no webfetch found, supported webfetch are: curl,
wget, prozilla, aria2

no metadata found for medium "various"
>>>

Whic results in:
<<<
Test Summary Report
-------------------
t/superuser--http.t                                           (Wstat:
2560 Tests: 11 Failed: 10)
  Failed tests:  2-11
  Non-zero exit status: 10
Files=40, Tests=3775, 953 wallclock secs ( 1.19 usr  0.08 sys + 309.33
cusr 52.13 csys = 362.73 CPU)
Result: FAIL
Failed 1/40 test programs. 10/3775 subtests failed.
make: *** [Makefile:1155: test_dynamic] Error 255
(/usr/bin/make test exited with 512)
CPAN::Reporter: Test result is 'fail', One or more tests failed.
>>>

And indeed:
$ rpm -q curl wget prozilla aria2
package curl is not installed
package wget is not installed
package prozilla is not installed
package aria2 is not installed

Should have been done in commit 360f05b8db2029cb62c11f7eddb7fb57f3b774f1

1 files changed, 3 insertions, 0 deletions

diff --git a/t/superuser--http.t b/t/superuser--http.t
index 15f0694a..297e7c69 100644
--- a/t/superuser--http.t
+++ b/t/superuser--http.t
@@ -8,6 +8,9 @@ use Cwd 'getcwd';
 
 
 need_root_and_prepare();
+
+need_downloader();
+
 my $url = start_httpd();
 
 my $name = 'various';