Errno test fails on Ubuntu 20.04

[robert-ancell]

Looks like a previous call failed with EINVAL:

test/errno_test.dart: errno (failed)
  Expected: a value equal to <0>
    Actual: <22>
     Which: is not a value equal to <0>

This is probably an issue in the Dart compiler or the test library, but is otherwise outside the control of this library.

[robert-ancell]

Disabling this test in https://github.com/canonical/stdlibc.dart/pull/115