MVP of a signal messenger test (#230)

* MVP of a signal messenger test

* Add minimal signal test unit tests

* Add Signal test to the im nettest group

* Add test for https://sfu.voip.signal.org/

* Fix bug in client-side determination of blocking status

* Add uptime.signal.org to the test targets

* Add more tests

* Check for invalid CA being passed
* Check that the update function works as expected

* Update internal/engine/experiment/signal/signal_test.go

Co-authored-by: Simone Basso <bassosimone@gmail.com>

* fix: back out URL we shouldn't have changed

When merging probe-engine into probe-cli, we changed too many URLs
and some of them should not have been changed.

I noticed this during the review of Signal and I choose to add
this commit to revert such changes.

While there, make sure the URL of the experiment is OK.

* fix(signal): reach 100% of coverage

Just so that we can focus on areas of the codebase where we need
more coverage, let us avoid missing an easy line to test.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/engine/netx/netx.go

@@ -17,7 +17,7 @@
 // TLS handshake for DoT and DoH).
 //
 // We described the design and implementation of the most recent version of
-// this package at <https://github.com/ooni/probe-cli/v3/internal/engine/issues/359>. Such
+// this package at <https://github.com/ooni/probe-engine/issues/359>. Such
 // issue also links to a previous design document.
 package netx