Make sure vcpkg_test_cmake uses the correct generator

Otherwise, on Windows, it will default to x86 based generator that will
fail finding x64 builds. For instance, when building for x64-windows:

CMake Error at CMakeLists.txt:4 (find_package):
  Could not find a configuration file for package "CapnProto" that is
  compatible with requested version "".

  The following configuration files were considered but not accepted:

    vcpkg/packages/capnproto_x64-windows/share/capnproto/CapnProtoConfig.cmake, version: 0.7.0 (64bit)

scripts/cmake/vcpkg_test_cmake.cmake

@@ -4,7 +4,7 @@
 ##
 ## ## Usage:
 ## ```cmake
-## vcpkg_test_cmake(PACKAGE_NAME <name> [MODULE])
+## vcpkg_test_cmake(PACKAGE_NAME <name> [MODULE] [GENERATOR generator])
 ## ```
 ##
 ## ## Parameters:
@@ -15,8 +15,12 @@
 ## ### MODULE
 ## Indicates that the library expects to be found via built-in CMake targets.
 ##
+## ### GENERATOR
+## The cmake generator to use for the test. It not provided, fallback to the one used
+## during a previous vcpkg_configure_cmake() call, or use CMake default generator.
+##
 function(vcpkg_test_cmake)
-    cmake_parse_arguments(_tc "MODULE" "PACKAGE_NAME" "" ${ARGN})
+    cmake_parse_arguments(_tc "MODULE" "PACKAGE_NAME;GENERATOR" "" ${ARGN})
 
     if(NOT DEFINED _tc_PACKAGE_NAME)
       message(FATAL_ERROR "PACKAGE_NAME must be specified")
@@ -27,6 +31,13 @@ function(vcpkg_test_cmake)
       set(PACKAGE_TYPE CONFIG)
     endif()
 
+    if(_tc_GENERATOR)
+      set(GENERATOR ${_tc_GENERATOR})
+    else()
+      # vcpkg_configure_cmake defines _VCPKG_CMAKE_GENERATOR when called
+      set(GENERATOR ${_VCPKG_CMAKE_GENERATOR})
+    endif()
+
     if(VCPKG_PLATFORM_TOOLSET STREQUAL "v142")
       message(STATUS "Skipping CMake integration test due to v142 / CMake interaction issues")
       return()
@@ -36,32 +47,21 @@ function(vcpkg_test_cmake)
     file(REMOVE_RECURSE ${CURRENT_BUILDTREES_DIR}/${TARGET_TRIPLET}-test)
     file(MAKE_DIRECTORY ${CURRENT_BUILDTREES_DIR}/${TARGET_TRIPLET}-test)
 
-    #Generate Dummy source
-#    set(VCPKG_TEST_SOURCE ${CURRENT_BUILDTREES_DIR}/${TARGET_TRIPLET}-test/CMakeIntegration.cpp)
-#    file(WRITE  ${VCPKG_TEST_SOURCE} "int main() \{\n")
-#    file(APPEND ${VCPKG_TEST_SOURCE} "return 0;}")
     # Generate test source CMakeLists.txt
     set(VCPKG_TEST_CMAKELIST ${CURRENT_BUILDTREES_DIR}/${TARGET_TRIPLET}-test/CMakeLists.txt)
     file(WRITE  ${VCPKG_TEST_CMAKELIST} "cmake_minimum_required(VERSION 3.10)\n")
     file(APPEND ${VCPKG_TEST_CMAKELIST} "set(CMAKE_PREFIX_PATH \"${CURRENT_PACKAGES_DIR};${CURRENT_INSTALLED_DIR}\")\n")
     file(APPEND ${VCPKG_TEST_CMAKELIST} "\n")
     file(APPEND ${VCPKG_TEST_CMAKELIST} "find_package(${_tc_PACKAGE_NAME} ${PACKAGE_TYPE} REQUIRED)\n")
-    #To properly test if the package is actually working haveway correctly we have to link all targets of a package to 
-    #a test executable and than actually build it. This will not discover if every symbol exported by the library is available/linked
-    #but it will doscover if all files which are linked by a target actual exist. Problem is: How to discover all targets?
-#    file(APPEND ${VCPKG_TEST_CMAKELIST} "add_executable(${_tc_PACKAGE_NAME}_exe ${VCPKG_TEST_SOURCE})\n")
-#    file(APPEND ${VCPKG_TEST_CMAKELIST} "target_link_libraries(${_tc_PACKAGE_NAME}_exe PRIVATE ${_tc_PACKAGE_NAME})\n")
 
-    if(DEFINED _VCPKG_CMAKE_GENERATOR)
-        set(VCPKG_CMAKE_TEST_GENERATOR "${_VCPKG_CMAKE_GENERATOR}")
-    else()
-        set(VCPKG_CMAKE_TEST_GENERATOR Ninja)
+    if(GENERATOR)
+      set(GENERATOR_ARGS "-G" "${GENERATOR}")
     endif()
 
     # Run cmake config with a generated CMakeLists.txt
     set(LOGPREFIX "${CURRENT_BUILDTREES_DIR}/test-cmake-${TARGET_TRIPLET}")
     execute_process(
-      COMMAND ${CMAKE_COMMAND} -G ${VCPKG_CMAKE_TEST_GENERATOR} .
+      COMMAND ${CMAKE_COMMAND} ${GENERATOR_ARGS} .
       OUTPUT_FILE "${LOGPREFIX}-out.log"
       ERROR_FILE "${LOGPREFIX}-err.log"
       RESULT_VARIABLE error_code