test/common: do not leak in MemoryIsZeroSmallTest

test/common: do not leak in MemoryIsZeroSmallTest

before this change, we allocate memory chunks with specified
size using `new []`, but we never free them. when testing with
LeakSanitizer enabled, it rightly points identifies the leakage:

```
Direct leak of 8754 byte(s) in 184 object(s) allocated from:
    #0 0x55c0b2470f0d in operator new[](unsigned long) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_memory+0x196f0d) (BuildId: d3267dd8819427b804c4729e0467dbe7601fb321)
    #1 0x55c0b247456c in MemoryIsZeroSmallTest_MemoryIsZeroTestSmall_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/common/test_memory.cc:33:18
    #2 0x55c0b2598ee6 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #3 0x55c0b2553b92 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #4 0x55c0b25049dc in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #5 0x55c0b2506a12 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #6 0x55c0b250804b in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #7 0x55c0b25254d8 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #8 0x55c0b25a16f6 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
    #9 0x55c0b255a502 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
    #10 0x55c0b2524862 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #11 0x55c0b24ab4c0 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #12 0x55c0b24ab451 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googlemock/src/gmock_main.cc:70:10
    #13 0x7f45e065ad8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
```

in this change, we free the allocate memory.

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