appveyor: use random test server ports based upon APPVEYOR_API_URL

Avoid conflicts of test server ports with AppVeyor API on localhost.

Closes #5034

appveyor.yml

@@ -249,7 +249,8 @@ build_script:
 
 test_script:
     - if %TESTING%==ON (
-        bash.exe -e -l -c "cd /c/projects/curl/tests && ./runtests.pl -a -p !flaky %DISABLED_TESTS%" )
+        echo APPVEYOR_API_URL=%APPVEYOR_API_URL% &&
+        bash.exe -e -l -c "cd /c/projects/curl/tests && ./runtests.pl -a -b$(($(echo '%APPVEYOR_API_URL%' | cut -d'/' -f3 | cut -d':' -f2)+1)) -p !flaky %DISABLED_TESTS%" )
 
 # whitelist branches to avoid testing feature branches twice (as branch and as pull request)
 branches: