fuzz_buffer.cpp failing flakily on appveyor: cannot find/write log dir

https://ci.appveyor.com/project/DynamoRIO/drmemory/build/1.0.140

```
36/53 Test #36: fuzz_buffer.cpp ...................***Failed 0.05 sec
STDOUT:
STDERR: ERROR: invalid -logdir: cannot find/write C:\projects\drmemory\build\build_drmemory-dbg-64\logs
CMake Error at C:/projects/drmemory/build/build_drmemory-dbg-64/tests/runtest.cmake:331 (message):
***
C:/projects/drmemory/build/build_drmemory-dbg-64/bin64/drmemory.exe;-debug;-dr_debug;-dr;C:/projects/drmemory/build/build_drmemory-dbg-64/dynamorio;-batch;-dr_ops;-dumpcore_mask;-dr_ops;0x87fd;-dr_ops;-stderr_mask;-dr_ops;15;-dr_ops;-msgbox_mask;-dr_ops;0;-callstack_style;0x27;-no_results_to_stderr;-no_fuzz_dump_on_error;-fuzz_function;?repeatme-BufferPrinter--QEAAXPEAI_K-Z;-fuzz_num_args;3;-fuzz_data_idx;1;-fuzz_size_idx;2;-fuzz_num_iters;10;-fuzz_mangled_names;--;C:/projects/drmemory/build/build_drmemory-dbg-64/tests/fuzz_buffer.cpp.exe;initialize
has the wrong exit code (1): ERROR: invalid -logdir: cannot find/write
C:\projects\drmemory\build\build_drmemory-dbg-64\logs
***
```