tests: print numeric error value too on failure

tests: print numeric error value too on failure

The error message is not always meaningful. Also, sometimes we care about the
sign of the value, and we ignore the sign of the error in the printing machinery.

The messages for errno are changed to say "errno" instead of "error". The problem with
the previous formalation is that our errors are always negative and errnos are
positive, so when we print the numerical value, we cannot use the word for both.

Example output:
src/test/test-tests.c:15: Assertion failed: Expected "-1" to succeed, but got error: -1/Operation not permitted
src/test/test-tests.c:16: Assertion failed: Expected "-ENOANO" to succeed, but got error: -55/No anode
src/test/test-tests.c:61: Assertion failed: Expected "0" to fail with error -2/"No such file or directory", but it succeeded
src/test/test-tests.c:62: Assertion failed: Expected "RET_NERRNO(mkdir("/i/will/fail/with/enoent", 666))" to fail with error -55/"No anode", but got the following error: -2/No such file or directory
src/test/test-tests.c:68: Assertion failed: Expected "0" to fail with errno 2/"No such file or directory", but it succeeded
src/test/test-tests.c:70: Assertion failed: Expected "mkdir("/i/will/fail/with/enoent", 666)" to fail with errno 55/"No anode", but got the following errno: 2/No such file or directory