Use os-stdlib in tests.

--- a/utils/build-presets.ini
+++ b/utils/build-presets.ini
@@ -872,7 +872,7 @@
 indexstore-db
 sourcekit-lsp
 swiftdocc
-lit-args=-v --time-tests
+lit-args=-v --time-tests --param use_os_stdlib
 
 # rdar://problem/31454823
 lldb-test-swift-only