[test] Echo all failures to the same file

Fix-up for 482d2ba. This slipped in while I was moving things
around, but fortunately that part of the test isn't currently failing.

Author: jrose-apple

validation-test/SIL/verify_all_overlays.sil

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: for x in %platform-sdk-overlay-dir/*.swiftmodule; do [[ $(basename "$x") = Swift.swiftmodule ]] && continue; llvm-bcanalyzer $x | %FileCheck %s || echo "$x (bcanalyzer)" >> %t.txt; %target-sil-opt -sdk %sdk -enable-sil-verify-all $x > /dev/null || echo "$x (sil-opt)" >> %t/failures.txt; done
+// RUN: for x in %platform-sdk-overlay-dir/*.swiftmodule; do [[ $(basename "$x") = Swift.swiftmodule ]] && continue; llvm-bcanalyzer $x | %FileCheck %s || echo "$x (bcanalyzer)" >> %t/failures.txt; %target-sil-opt -sdk %sdk -enable-sil-verify-all $x > /dev/null || echo "$x (sil-opt)" >> %t/failures.txt; done
 // RUN: not cat %t/failures.txt
 
 // CHECK-NOT: Unknown