Fix to CMakeLists for spirv-fuzz tests (#2888)

A previous change that disabled long-running tests by default failed
to enable short-running tests when long-running tests are enabled.
This change fixes that problem.
diff --git a/test/fuzz/CMakeLists.txt b/test/fuzz/CMakeLists.txt
index 715de63..1d9a8fd 100644
--- a/test/fuzz/CMakeLists.txt
+++ b/test/fuzz/CMakeLists.txt
@@ -42,7 +42,7 @@
     # spirv-fuzz bugs in changes that are totally unrelated to spirv-fuzz,
     # which would be counfounding.  Instead, they should be run regularly but
     # separately.
-    set(SOURCES
+    set(SOURCES ${SOURCES}
             fuzzer_replayer_test.cpp
             fuzzer_shrinker_test.cpp)
   endif()