silence backtrace warning

Author: hnrklssn

test/Interop/ObjC/swiftify-import/counted-by-protocol.swift

@@ -4,7 +4,7 @@
 // RUN: split-file %s %t
 
 // RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/CountedByProtocol.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers %t/counted-by-protocol.swift -verify -Xcc -Wno-nullability-completeness
-// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/CountedByProtocol.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers %t/counted-by-protocol.swift -dump-macro-expansions 2> %t/expansions.out
+// RUN: env SWIFT_BACKTRACE="" %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/CountedByProtocol.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers %t/counted-by-protocol.swift -dump-macro-expansions 2> %t/expansions.out
 // RUN: %diff %t/expansions.out %t/expansions.expected
 
 //--- Inputs/module.modulemap