SSL.Constants.ConnectionState: Added some composite values.

Adds some constants for common composite values, and updates
SSL.Connection to use them.
Testsuite: Support SRCDIR substitution in lib testsuites too.
Testsuite: Moved the async TLS close tests.

The async TLS close tests are now run from the main SSL testsuite,
to avoid missing them when running restricted tests.

FIXME: Consider moving the async_tls_close_test.pike script
somewhere else (Tools.Standalone?).