sess: try sysroot candidates for fluent bundle

Instead of checking only the user provided sysroot or the default (when
no sysroot is provided), search user provided sysroot and then check
default sysroots for locale requested by the user.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_interface/src/tests.rs

@@ -44,6 +44,7 @@ fn mk_session(matches: getopts::Matches) -> (Session, CfgSpecs) {
     let sess = build_session(
         sessopts,
         None,
+        None,
         registry,
         DiagnosticOutput::Default,
         Default::default(),