test/osd/hitset: free allocated HitSet

test/osd/hitset: free allocated HitSet

we allocate a hitset without freeing it in this test, and LeakSanitizer
points this out
```
Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x557a0841a9dd in operator new(unsigned long) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_hitset+0x1ae9dd) (BuildId: ad9be2b52b3d6fb1a567b262c3becaab6373e88d)
    #1 0x557a0843b98e in ExplicitHashHitSetTest::ExplicitHashHitSetTest() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/osd/hitset.cc:128:46
    #2 0x557a0843b918 in ExplicitHashHitSetTest_Construct_Test::ExplicitHashHitSetTest_Construct_Test() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/osd/hitset.cc:133:1
    #3 0x557a0843b8cb in testing::internal::TestFactoryImpl<ExplicitHashHitSetTest_Construct_Test>::CreateTest() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/internal/gtest-internal.h:472:44
    #4 0x557a08532406 in testing::Test* testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::TestFactoryBase, testing::Test*>(testing::internal::TestFactoryBase*, testing::Test* (testing::internal::TestFactoryBase::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #5 0x557a084eb892 in testing::Test* testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::TestFactoryBase, testing::Test*>(testing::internal::TestFactoryBase*, testing::Test* (testing::internal::TestFactoryBase::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #6 0x557a0849dd55 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2848:22
    #7 0x557a0849f3bb in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #8 0x557a084bc848 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #9 0x557a0853a6d6 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #10 0x557a084f1222 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #11 0x557a084bbbd2 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #12 0x557a08441c10 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #13 0x557a08441ba1 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googlemock/src/gmock_main.cc:70:10
    #14 0x7faa493d6d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
```

in this change, we just free it in the dtor. this should address
the warning from the sanitizer.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>