Rename long test names to be a bit shorter

Helps to avoid hitting path limits on Windows

src/test/run-pass-fulldeps/issue-16723.rs

@@ -9,9 +9,9 @@
 // except according to those terms.
 
 // ignore-stage1
-// aux-build:issue_16723_multiple_items_syntax_ext.rs
+// aux-build:issue-16723.rs
 #![feature(plugin)]
-#![plugin(issue_16723_multiple_items_syntax_ext)]
+#![plugin(issue_16723)]
 
 multiple_items!();
 

src/test/run-pass-fulldeps/outlive-expansion-phase.rs

@@ -8,10 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// aux-build:plugin_crate_outlive_expansion_phase.rs
+// aux-build:outlive-expansion-phase.rs
 // ignore-stage1
 
 #![feature(plugin)]
-#![plugin(plugin_crate_outlive_expansion_phase)]
+#![plugin(outlive_expansion_phase)]
 
 pub fn main() {}

src/test/run-pass/specialization/cross-crate-defaults.rs

@@ -8,13 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// aux-build:specialization_cross_crate_defaults.rs
+// aux-build:cross_crates_defaults.rs
 
 #![feature(specialization)]
 
-extern crate specialization_cross_crate_defaults;
+extern crate cross_crates_defaults;
 
-use specialization_cross_crate_defaults::*;
+use cross_crates_defaults::*;
 
 struct LocalDefault;
 struct LocalOverride;