test: make sure we retry the auth callback on all platforms

We were missing this test on Windows, which meant we didn't notice that
we never fixed the single authentication attempt it tries, nor its wrong
return code.

Enable this for the unix platforms as well over HTTP. We previously were
doing it locally but disabled it on OS X due to issues with its sshd not
accepting password authentication.

appveyor.yml

@@ -36,4 +36,8 @@ build_script:
 - cmd: |
     if "%GENERATOR%"=="MSYS Makefiles" (C:\MinGW\msys\1.0\bin\sh --login /c/projects/libgit2/script/appveyor-mingw.sh)
 test_script:
-- ps: ctest -V .
+- ps: |
+    ctest -V .
+    $env:GITTEST_REMOTE_URL="https://github.com/libgit2/non-existent"
+    $env:GITTEST_REMOTE_USER="libgit2test"
+    .\Debug\libgit2_clar -sonline::clone::cred_callback

script/cibuild.sh

@@ -56,3 +56,7 @@ if [ -e ./libgit2_clar ]; then
         ./libgit2_clar -sonline::clone::cred_callback || exit $?
     fi
 fi
+
+export GITTEST_REMOTE_URL="https://github.com/libgit2/non-existent"
+export GITTEST_REMOTE_USER="libgit2test"
+./libgit2_clar -sonline::clone::cred_callback || exit $?