[Test] Scribble on runtime allocations by default.

Author: nate-chandler

test/lit.cfg

@@ -1912,6 +1912,9 @@ if config.lldb_build_root != "":
 # variable.
 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'
+
 # 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'