test: Fix to initialize the buffer

test: Fix to initialize the buffer

In src/test/test_diagnostics_data_write.c, test functions are using char
array(256-byte length) as a buffer but they did not initialized.

Because of this uninitialized buffers, errors were raised by the gcc-13
compiler like below:
	error: 'buf' may be used uninitialized
To fix this issue, buffers were initialized when they are declared.

Change-Id: If69983ddce58be07b3269741acd4ba9fba587936
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>