Tests: Fix RunCMake.GeneratorExpression to run in CMake 3.9

CMake 3.9 does not have CMP0070, so port away from it for that branch.

Tests/RunCMake/GeneratorExpression/TARGET_PROPERTY-SOURCES.cmake

@@ -1,5 +1,5 @@
-cmake_policy(SET CMP0070 NEW)
 enable_language(C)
 add_library(foo empty.c empty2.c)
 target_sources(foo PRIVATE empty3.c)
-file(GENERATE OUTPUT foo.txt CONTENT "$<TARGET_PROPERTY:foo,SOURCES>")
+file(GENERATE OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/foo.txt
+     CONTENT "$<TARGET_PROPERTY:foo,SOURCES>")