[Backtracing] Disable backtracer for Runtime/backtrace test.

The Runtime/backtrace test is a test of the fatalError() backtracer,
which gets turned off when we have the new backtracer enabled.  So,
to make this test work, we need to turn off the new backtracer.

rdar://117470489

Author: al45tair

test/Runtime/backtrace.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-build-swift %s -o %t/a.out
-// RUN: %{python} %S/../Inputs/not.py "%target-run %t/a.out" 2>&1 | %{python} %utils/backtrace-check
+// RUN: env SWIFT_BACKTRACE=enable=no %{python} %S/../Inputs/not.py "%target-run %t/a.out" 2>&1 | %{python} %utils/backtrace-check
 
 // NOTE: not.py is used above instead of "not --crash" because %target-run
 // doesn't pass through the crash, and `not` may not be available when running