test/erasure-code: fix memory leak in ErasureCodePlugin.parity_delta_write

test/erasure-code: fix memory leak in ErasureCodePlugin.parity_delta_write

Fix 4KB memory leak in ErasureCodePlugin_parity_delta_write_Test caused by
unmanaged raw buffer allocation. The test was allocating a 4096-byte raw
buffer to replace shard 4 for delta encoding validation, but the buffer::ptr
constructed from the raw pointer did not manage the buffer's lifecycle.

Detected by AddressSanitizer:

```
Direct leak of 4096 byte(s) in 1 object(s) allocated from:
    #0 0x7fb73a720e15 in malloc /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:67
    #1 0x5562f4062ccc in ErasureCodePlugin_parity_delta_write_Test::TestBody() /home/kefu/dev/ceph/src/test/erasure-code/TestErasureCodePluginJerasure.cc:122
    #2 0x5562f41081a1 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2653
    #3 0x5562f40f3004 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2689
    #4 0x5562f409cbba in testing::Test::Run() /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2728```
```

In this change, we replace raw pointer allocation with
create_bufferptr() to ensure proper memory management by buffer::ptr.

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