[Test] Fix backtrace.swift to use %target-run.

rdar://problem/50863395

Author: mikeash

test/Runtime/backtrace.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-build-swift %s -o %t/a.out
-// RUN: not --crash %t/a.out 2>&1 | %{python} %utils/backtrace-check
+// RUN: not --crash %target-run %t/a.out 2>&1 | %{python} %utils/backtrace-check
 
 // This is not supported on watchos, ios, or tvos
 // UNSUPPORTED: OS=watchos