Make the skip list corpus tests opt-in, not default

Review feedback: running the skip list tests on every default ctest run is a
waste of time. Gate them behind a TEST_SKIP_LIST CMake option, off by default,
so they remain easy to run when cross-checking SkipList.cpp against
ReferenceImpl.

Verified with -DTEST_SKIP_LIST=ON: all 2845 skip_list corpus tests pass; the
default test list is unchanged.

CMakeLists.txt

@@ -182,6 +182,8 @@ endif()
 
 include(CTest)
 
+option(TEST_SKIP_LIST "Run the slow skip list cross-check tests" OFF)
+
 # disable tests if this is being used through e.g. FetchContent
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
 
@@ -224,11 +226,14 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
   target_compile_options(driver_skip_list PRIVATE ${TEST_FLAGS})
   target_link_libraries(driver_skip_list PRIVATE skip_list)
 
-  # skip list tests, which cross-check SkipList.cpp against ReferenceImpl
-  foreach(TEST ${CORPUS_TESTS})
-    get_filename_component(hash ${TEST} NAME)
-    add_test(NAME skip_list_${hash} COMMAND driver_skip_list ${TEST})
-  endforeach()
+  # Optional cross-check of SkipList.cpp against ReferenceImpl. Opt-in via
+  # -DTEST_SKIP_LIST=ON, since these are slow.
+  if(TEST_SKIP_LIST)
+    foreach(TEST ${CORPUS_TESTS})
+      get_filename_component(hash ${TEST} NAME)
+      add_test(NAME skip_list_${hash} COMMAND driver_skip_list ${TEST})
+    endforeach()
+  endif()
 
   # ad hoc testing
   add_executable(conflict_set_main ConflictSet.cpp)