tests: print errno name rather than the message

tests: print errno name rather than the message

The tests are written and consumed by developers. Errno descriptions are good
for users, but for developers the errno "name" is actually more useful, and
we need to always map the description back to the name to compare with the
code. Let's make things simpler for ourselves by printing the errno names
directly.

Example output:

src/test/test-tests.c:15: Assertion failed: Expected "-1" to succeed, but got error: -1/EPERM
src/test/test-tests.c:16: Assertion failed: Expected "-ENOANO" to succeed, but got error: -55/ENOANO
src/test/test-tests.c:20: Assertion failed: Expected "0" to be positive, but it is zero.
src/test/test-tests.c:62: Assertion failed: Expected "RET_NERRNO(mkdir("/i/will/fail/with/enoent", 666))" to fail with error -55/ENOANO, but got -2/ENOENT
src/test/test-tests.c:68: Assertion failed: Expected "0" to fail with errno 2/ENOENT, but it succeeded
src/test/test-tests.c:70: Assertion failed: Expected "mkdir("/i/will/fail/with/enoent", 666)" to fail with errno 55/ENOANO, but got 2/ENOENT