Add test cases for SECTION UNION defining multiple identical labels (#1349)

Exported labels should fail to link; non-exported ones should be okay.

test/CMakeLists.txt

@@ -2,12 +2,18 @@
 
 add_executable(randtilegen gfx/randtilegen.cpp)
 add_executable(rgbgfx_test gfx/rgbgfx_test.cpp)
+add_executable(unmangle link/unmangle.cpp)
 
 install(TARGETS randtilegen rgbgfx_test
         DESTINATION ${rgbds_SOURCE_DIR}/test/gfx
         COMPONENT "Test support programs"
         EXCLUDE_FROM_ALL
         )
+install(TARGETS unmangle
+        DESTINATION ${rgbds_SOURCE_DIR}/test/link
+        COMPONENT "Test support programs"
+        EXCLUDE_FROM_ALL
+        )
 
 foreach(TARGET randtilegen rgbgfx_test)
   if(LIBPNG_FOUND) # pkg-config