tests: disable COW checking by default, and enable it for specific tests

COW checking needs that all libraries are consistently compiled with asserts enabled. This is not the case for the Foundation overlay anymore.
Therefore it does not work with some tests which interop with Foundation.

The solution here is to disable COW checking by default, but enable it for Array tests which do not interop with Foundation.

(cherry picked from commit 0831240)

Author: eeckstein

test/lit.cfg

@@ -2135,6 +2135,13 @@ config.environment['SWIFT_DEBUG_ENABLE_COW_CHECKS'] = 'false'
 # Add this to the command which runs an executable to enable COW checks in the swift runtime.
 config.substitutions.append(('%enable-cow-checking', TARGET_ENV_PREFIX + 'SWIFT_DEBUG_ENABLE_COW_CHECKS=true;'))
 
+# Disabe COW sanity checks in the swift runtime by default.
+# (But it's required to set this environment variable to something)
+config.environment['SWIFT_DEBUG_ENABLE_COW_CHECKS'] = 'false'
+
+# Add this to the command which runs an executable to enable COW checks in the swift runtime.
+config.substitutions.append(('%enable-cow-checking', TARGET_ENV_PREFIX + 'SWIFT_DEBUG_ENABLE_COW_CHECKS=true;'))
+
 if config.lldb_build_root != "":
     lldb_python_path = get_lldb_python_path(config.lldb_build_root)
     lldb_python_interpreter = get_lldb_python_interpreter(config.lldb_build_root)