[libc++] Enable tests for the experimental library by default

This matches the fact that we build the experimental library by default.
Otherwise, by default we'd be building the library but not testing it,
which is inconsistent.

Differential Revision: https://reviews.llvm.org/D102109

GitOrigin-RevId: 875ff8e059b73f65f5ad08d416a0a9caf0673d1a
diff --git a/test/lit.site.cfg.in b/test/lit.site.cfg.in
index 14fabce..51b4631 100644
--- a/test/lit.site.cfg.in
+++ b/test/lit.site.cfg.in
@@ -29,6 +29,7 @@
 config.sysroot                  = "@LIBUNWIND_SYSROOT@"
 config.gcc_toolchain            = "@LIBUNWIND_GCC_TOOLCHAIN@"
 config.cxx_ext_threads          = @LIBUNWIND_BUILD_EXTERNAL_THREAD_LIBRARY@
+config.enable_experimental      = False
 
 site.addsitedir(os.path.join(config.libunwind_src_root, 'test'))
 site.addsitedir(os.path.join(config.libcxx_src_root, 'utils'))