Linux sandbox: Provide AssertSingleThreaded() helper

sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc

Index: sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc
diff --git a/sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc b/sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc
index 40a99e4b6abd22a875e4c09c95c5caa5ac254f31..7ddf6fb5f37293fad321d2b7f4c7453eba99b8b0 100644
--- a/sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc
+++ b/sandbox/linux/bpf_dsl/bpf_dsl_more_unittest.cc
@@ -43,6 +43,7 @@
 #include "sandbox/linux/seccomp-bpf/trap.h"
 #include "sandbox/linux/services/linux_syscalls.h"
 #include "sandbox/linux/services/syscall_wrappers.h"
+#include "sandbox/linux/services/thread_helpers.h"
 #include "sandbox/linux/syscall_broker/broker_file_permission.h"
 #include "sandbox/linux/syscall_broker/broker_process.h"
 #include "sandbox/linux/tests/scoped_temporary_file.h"
@@ -2308,7 +2309,8 @@ class AllowAllPolicy : public Policy {
 SANDBOX_DEATH_TEST(
     SandboxBPF,
     StartMultiThreadedAsSingleThreaded,
-    DEATH_MESSAGE("Cannot start sandbox; process is already multi-threaded")) {
+    DEATH_MESSAGE(
+        ThreadHelpers::GetAssertSingleThreadedErrorMessageForTests())) {
   base::Thread thread("sandbox.linux.StartMultiThreadedAsSingleThreaded");
   BPF_ASSERT(thread.Start());