test: fix error message tests on non-glibc based systems

test: fix error message tests on non-glibc based systems

Our tests makes strict assumptions on the error messages generated by
strerror(3P), but those aren't in any way required to have a special
text. This works just fine on glibc based systems, but causes us to
trigger asserts on other systems like e.g. musl based ones.

Fix this by introducing a new `assertErrorMessage` method. It'll take as
inputs the observed error message as well as the error code we expect
and then use `os.strerror` to compute the expected message. Like this,
we're not dependent anymore on how the system represents those error
messages but generate the expected ones on the fly.