appveyor: test tidy-ups

- wait for sshd before the build, to ease testing.
- restore the RSA 2048 hostkey generator line, commented, to ease tests.
- keep per-job `SKIP_CTEST` settings, commented, to ease tests.
- drop automatism to disable tests for ARM64, for simplicity.

Note: This has been tested with tests enabled, but they remain
a hit-or-miss (mostly a miss) due to failed sshd connection, and this
patch does not re-enable them.

FTR:
- a CI run when tests worked without issues (2023-12-04):
  https://ci.appveyor.com/project/libssh2org/libssh2/builds/48673013/job/79lywpm3pfvq7hn5
- a fresh CI run when the first job successfully waited for sshd:
  https://ci.appveyor.com/project/libssh2org/libssh2/builds/53941135/job/yu9jf5hvp3grse0g

Ref: #1856 #1857
Follow-up to c71ff14f3b #1894
Follow-up to c321b324d6 #1768
Follow-up to b5e68bdc37 #1461
Follow-up to f36edf94e7 #997

Closes #1892

scripts/appveyor/docker-bridge.sh

@@ -7,6 +7,7 @@ set -eu
 netsh interface portproxy add v4tov4 listenport=3389 listenaddress="$1" connectport=22 connectaddress=127.0.0.1
 netsh interface portproxy show all
 
+# ssh-keygen -b 2048 -t rsa -f auth -q -N '' && mkdir .ssh && mv auth.pub .ssh/authorized_keys
 ssh-keygen -t ed25519 -f auth -q -N '' && mkdir .ssh && mv auth.pub .ssh/authorized_keys
 ssh-keygen -A
 "$(command -v sshd)" &