tests: by default enable COW sanity checks for all tests

Author: eeckstein

test/lit.cfg

@@ -1940,6 +1940,9 @@ config.environment[TARGET_ENV_PREFIX + 'SWIFT_DETERMINISTIC_HASHING'] = '1'
 # Enable malloc scribble during tests by default.
 config.environment[TARGET_ENV_PREFIX + 'SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE'] = 'YES'
 
+# Enable COW sanity checks in the swift runtime by default.
+config.environment['SWIFT_DEBUG_ENABLE_COW_SANITY_CHECKS'] = 'true'
+
 # Run lsb_release on the target to be tested and return the results.
 def linux_get_lsb_release():
     lsb_release_path = '/usr/bin/lsb_release'