tests: ec_host_cmd: Make sure expected_dut_to_host fits in buffer

If struct tx_structure is too large than the buffer that
expected_dut_to_host references, leading the compiler to generate a warning
about the possibility of buffer overflow.

Leave out the 'struct ec_response_too_big' so that 'struct tx_structure'
fits within the 256 byte buffer.

This was caught when using GCC 11.3.0 building the test for native_posix.

Signed-off-by: Keith Packard <keithp@keithp.com>