merge: improve Buffer tests

merge: improve Buffer tests

 * test: improve test_mutt_buffer_add_printf()
 * test: improve test_mutt_buffer_addch()
 * test: improve test_mutt_buffer_addstr()
 * test: improve test_mutt_buffer_addstr_n()
 * test: improve test_mutt_buffer_alloc()
 * test: improve test_mutt_buffer_concat_path()
 * test: improve test_mutt_buffer_fix_dptr()
 * test: improve test_mutt_buffer_free()
 * test: improve test_mutt_buffer_from()
 * test: improve test_mutt_buffer_increase_size()
 * test: improve test_mutt_buffer_init()
 * test: improve test_mutt_buffer_is_empty()
 * test: improve test_mutt_buffer_len()
 * test: improve test_mutt_buffer_new()
 * test: improve test_mutt_buffer_printf()
 * test: improve test_mutt_buffer_reset()
 * test: improve test_mutt_buffer_strcpy()
 * test: improve test_mutt_buffer_strcpy_n()
 * reorganise buffer header