[runtimes] Officially deprecate the legacy testing configuration system

Add a warning and tweak the release note to explain that the deprecation
targets libc++, libc++abi and libuwnind as well.

Also, as a fly-by, ensure that our CI runs the legacy testing configuration
for libc++, libc++abi and libunwind. This doesn't matter too much since
it's deprecated, but we might as well test it properly.

Differential Revision: https://reviews.llvm.org/D126478

GitOrigin-RevId: 719bf2d9d9fdb8792b15d93364c9c6b2ddd68a36
diff --git a/test/lit.site.cfg.in b/test/lit.site.cfg.in
index ae387f5..7a786a3 100644
--- a/test/lit.site.cfg.in
+++ b/test/lit.site.cfg.in
@@ -60,3 +60,6 @@
 configuration = libcxxabi.test.config.Configuration(lit_config, config)
+lit_config.warning("This is a legacy testing configuration which will be removed in LLVM 16. "
+                   "Please use one of the configurations in libcxxabi/test/configs or define your own.")