tablet_server-test: avoid NO_FATALS gtest race
tablet_server-test: avoid NO_FATALS gtest race

We saw a failure of the test with the following stack trace:

*** Aborted at 1586816377 (unix time) try "date -d @1586816377" if you are using GNU date ***
PC: @     0x7f0b851df05a testing::Test::HasFatalFailure()
*** SIGSEGV (@0x7f0b6be4e010) received by PID 8291 (TID 0x7f0b85cbb400) from PID 1810161680; stack trace: ***
    @           0x4be38d __tsan::CallUserSignalHandler() at /home/jenkins-slave/workspace/kudu-master/3/thirdparty/src/llvm-9.0.0.src/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:1916
    @           0x4bf47a rtl_sigaction() at /home/jenkins-slave/workspace/kudu-master/3/thirdparty/src/llvm-9.0.0.src/projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:2006
    @     0x7f0b7f999330 (unknown) at ??:0
    @     0x7f0b851df05a testing::Test::HasFatalFailure() at ??:0
    @           0x574687 kudu::tserver::TabletServerMaintenanceMemoryPressureTest_TestDontStarveDMSWhileUnderMemoryPressure_Test::TestBody() at /home/jenkins-slave/workspace/kudu-master/3/src/kudu/tserver/tablet_server-test.cc:?
    @     0x7f0b851fffa0 testing::internal::HandleExceptionsInMethodIfSupported<>() at ??:0
    @     0x7f0b851def39 testing::Test::Run() at ??:0
    @     0x7f0b851e01dd testing::TestInfo::Run() at ??:0
    @     0x7f0b851e0ef7 testing::TestCase::Run() at ??:0
    @     0x7f0b851ed3a7 testing::internal::UnitTestImpl::RunAllTests() at ??:0
    @     0x7f0b85200e50 testing::internal::HandleExceptionsInMethodIfSupported<>() at ??:0
    @     0x7f0b851ecc62 testing::UnitTest::Run() at ??:0
    @     0x7f0b8565c7bc RUN_ALL_TESTS() at ??:0
    @     0x7f0b8565b637 main at ??:0
    @     0x7f0b77360f45 __libc_start_main at ??:0
    @           0x4b6b75 (unknown) at ??:?

I haven't been able to reproduce this, but it seems like NO_FATALS calls
from a non-main thread may not always play nicely with an
ASSERT_EVENTUALLY in the main thread, which attempts to capture
assertion failures.

Change-Id: Icbb1a785606df93042c352ba350e31e9e828c8a9
Reviewed-on: http://gerrit.cloudera.org:8080/15734
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu