Ignore thread not suspended warning messages in LSAN

When a process with multiple threads is forked, LSAN outputs warning
messages to stderr like:

    ==276855==Running thread 276851 was not suspended. False leaks are possible.

We should ignore messages like this in tests.

Author: peterzhu2118

tool/lib/core_assertions.rb

@@ -328,6 +328,13 @@ def assert_separately(args, file = nil, line = nil, src, ignore_stderr: nil, **o
         args.insert((Hash === args.first ? 1 : 0), "-w", "--disable=gems", *$:.map {|l| "-I#{l}"})
         args << "--debug" if RUBY_ENGINE == 'jruby' # warning: tracing (e.g. set_trace_func) will not capture all events without --debug flag
         stdout, stderr, status = EnvUtil.invoke_ruby(args, src, capture_stdout, true, **opt)
+
+        if Test::Sanitizers.lsan_enabled?
+          # LSAN may output messages like the following line into stderr. We should ignore it.
+          #   ==276855==Running thread 276851 was not suspended. False leaks are possible.
+          # See https://github.com/google/sanitizers/issues/1479
+          stderr.gsub!(/==\d+==Running thread \d+ was not suspended\. False leaks are possible\.\n/, "")
+        end
       ensure
         if res_c
           res_c.close