Detect failures in forked function

EXPECT*/ASSERT* functions are not async-signal-safe and should not
be called from the function passed to InForkedProcess. However,
these happen accidentally sometimes but do no cause InForkedProcess
to fail.

Detect and notify in such cases.

PiperOrigin-RevId: 353354540

test/util/BUILD

@@ -172,6 +172,7 @@ cc_library(
         ":posix_error",
         ":save_util",
         ":test_util",
+        gtest,
         "@com_google_absl//absl/strings",
     ],
 )

test/util/multiprocess_util.cc

@@ -154,6 +154,9 @@ PosixErrorOr<int> InForkedProcess(const std::function<void()>& fn) {
   pid_t pid = fork();
   if (pid == 0) {
     fn();
+    TEST_CHECK_MSG(!::testing::Test::HasFailure(),
+                   "EXPECT*/ASSERT* failed. These are not async-signal-safe "
+                   "and must not be called from fn.");
     _exit(0);
   }
   MaybeSave();