[ROS2] Fix setting of diagnostic status level by diagnostic updater

[BasVolkers]

This PR resolves #183

It replaces every byte creation b'0' by b'\x00', b'1' by b'\x01', and b'2' by b'\x02'.
With these changes the diagnostic_aggregator_node no longer produces an ERROR.

[BasVolkers]

@Karsten1987

[Karsten1987]

@BasVolkers this looks to me as if we most likely don't test this correctly. Do you have an idea if we could enhance the test suite with some regression test for this change?

[Karsten1987]

• Linux
• Linux-aarch64
• macOS
• Windows

[BasVolkers]

@BasVolkers this looks to me as if we most likely don't test this correctly. Do you have an idea if we could enhance the test suite with some regression test for this change?

The tests only check that a certain value is set for the level of a DiagnosticStatus. Since the wrong values were used in the expected part of the assertEquals the tests still passed.

The actual error happens when the DiagnosticStatus's level is converted in status_item.hpp.
Since b'0' gives the ASCII value of '0' (48), the conversion fails. This fix make sure the level is in range [0-3].
A better fix may be using the DiagnosticStatus.msg levels (OK, WARN, ERROR), instead of creating the byte by hand.

I'm opening up MR #193 for this.