util/string_buffer: fix warning in tests

util/string_buffer: fix warning in tests

And also specify the maximum size when writing to static buffers. The
warning below refers to the case where "str5" could be larger than
"str5 - str4", then the strcat would have overlapping dst and src.

Compiler doesn't pick up the bound from the snprintf above, so we make
clear the bounds of str5 by using strncat() instead of strcat().

../../src/util/tests/string_buffer/string_buffer_test.cpp: In member function ‘virtual void string_buffer_string_buffer_tests_Test::TestBody()’:
../../src/util/tests/string_buffer/string_buffer_test.cpp:106:10: warning: ‘char* strcat(char*, const char*)’ accessing 81 or more bytes at offsets 48 and 128 may overlap 1 byte at offset 128 [-Wrestrict]
    strcat(str4, str5);
    ~~~~~~^~~~~~~~~~~~

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>