Tests: Restore PLplot contract test repository https URL

Switch back to the `https://` repository URL but without the `.git`
extensions.  It seems faster than the `git://` protocol for this server.

Tests/Contracts/PLplot/CMakeLists.txt

@@ -9,7 +9,7 @@ if(NOT PLplot_GIT_TAG)
   set(PLplot_GIT_TAG "plplot-5.13.0")
 endif()
 ExternalProject_Add(PLplot
-  GIT_REPOSITORY "git://git.code.sf.net/p/plplot/plplot"
+  GIT_REPOSITORY "https://git.code.sf.net/p/plplot/plplot"
   GIT_TAG "${PLplot_GIT_TAG}"
   PREFIX "${PLplot_PREFIX}"
   CMAKE_ARGS