Test cache entry property "STRINGS"

The STRINGS property tells cmake-gui to create a drop-down selection
list.  This teaches the Properties test to set and verify its value.

Tests/Properties/CMakeLists.txt

@@ -100,7 +100,7 @@ endif(NOT RESULT4)
 
 # test CACHE properties
 macro(check_cache_props)
-  foreach(prop VALUE TYPE HELPSTRING ADVANCED)
+  foreach(prop VALUE TYPE HELPSTRING ADVANCED STRINGS)
     get_property(result CACHE SOME_ENTRY PROPERTY ${prop})
     if(NOT "x${result}" STREQUAL "x${expect_${prop}}")
       message(SEND_ERROR "CACHE property ${prop} is [${result}], not [${expect_${prop}}]")
@@ -111,15 +111,19 @@ set(expect_VALUE "ON")
 set(expect_TYPE "BOOL")
 set(expect_HELPSTRING "sample cache entry")
 set(expect_ADVANCED 0)
+set(expect_STRINGS "")
 set(SOME_ENTRY "${expect_VALUE}" CACHE ${expect_TYPE} "${expect_HELPSTRING}" FORCE)
 mark_as_advanced(CLEAR SOME_ENTRY)
+set_property(CACHE SOME_ENTRY PROPERTY STRINGS "")
 check_cache_props()
 set(expect_VALUE "Some string")
 set(expect_TYPE "STRING")
 set(expect_HELPSTRING "sample cache entry help")
 set(expect_ADVANCED 1)
+set(expect_STRINGS "Some string;Some other string;Some third string")
 set_property(CACHE SOME_ENTRY PROPERTY TYPE "${expect_TYPE}")
 set_property(CACHE SOME_ENTRY PROPERTY HELPSTRING "${expect_HELPSTRING}")
 set_property(CACHE SOME_ENTRY PROPERTY VALUE "${expect_VALUE}")
 set_property(CACHE SOME_ENTRY PROPERTY ADVANCED "${expect_ADVANCED}")
+set_property(CACHE SOME_ENTRY PROPERTY STRINGS "${expect_STRINGS}")
 check_cache_props()