FindPkgConfig: Add test for specified pkg-config tool missing

Tests/RunCMake/FindPkgConfig/PkgConfigDoesNotExist-stdout.txt

+-- Could NOT find PkgConfig \(missing: PKG_CONFIG_EXECUTABLE\) *
+    Reason given by package: The command
+      "pkg-config-does-not-exist" --version
+    failed with output.*
+-- PKG_CONFIG_FOUND='FALSE'
+-- PKG_CONFIG_EXECUTABLE=''

Tests/RunCMake/FindPkgConfig/PkgConfigDoesNotExist.cmake

+set(PKG_CONFIG_EXECUTABLE "pkg-config-does-not-exist" CACHE FILEPATH "")
+find_package(PkgConfig)
+message(STATUS "PKG_CONFIG_FOUND='${PKG_CONFIG_FOUND}'")
+message(STATUS "PKG_CONFIG_EXECUTABLE='${PKG_CONFIG_EXECUTABLE}'")

Tests/RunCMake/FindPkgConfig/RunCMakeTest.cmake

 include(RunCMake)
 
+run_cmake(PkgConfigDoesNotExist)
+
 run_cmake(FindPkgConfig_NO_PKGCONFIG_PATH)
 run_cmake(FindPkgConfig_PKGCONFIG_PATH)
 run_cmake(FindPkgConfig_PKGCONFIG_PATH_NO_CMAKE_PATH)