Improve error message for tests with panic=abort

src/libsyntax_ext/test_harness.rs

@@ -67,7 +67,8 @@ pub fn inject(
                 PanicStrategy::Unwind
             }
             (PanicStrategy::Abort, false) => {
-                span_diagnostic.err("building tests with panic=abort is not yet supported");
+                span_diagnostic.err("building tests with panic=abort is not supported \
+                                     without `-Zpanic_abort_tests`");
                 PanicStrategy::Unwind
             }
             (PanicStrategy::Unwind, _) => PanicStrategy::Unwind,

src/test/ui/test-panic-abort-disabled.rs

@@ -1,4 +1,4 @@
-// error-pattern:building tests with panic=abort is not yet supported
+// error-pattern:building tests with panic=abort is not supported
 // no-prefer-dynamic
 // compile-flags: --test -Cpanic=abort
 // run-flags: --test-threads=1

src/test/ui/test-panic-abort-disabled.stderr

@@ -1,4 +1,4 @@
-error: building tests with panic=abort is not yet supported
+error: building tests with panic=abort is not supported without `-Zpanic_abort_tests`
 
 error: aborting due to previous error