tests: don't use -Zorbit on run-pass/issue-28950, it stack overflows.

Author: eddyb

src/test/run-pass/issue-28950.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -Z orbit=off
+// (blows the stack with MIR trans and no optimizations)
+
 // Tests that the `vec!` macro does not overflow the stack when it is
 // given data larger than the stack.