lib/libkvm: start adding basic tests for kvm(3)

lib/libkvm: start adding basic tests for kvm(3)

- kvm_close: add a testcase to verify support for errno = EINVAL / -1
  (see D10065) when kd == NULL is provided to the libcall.
- kvm_geterr:
-- Add a negative testcase for kd == NULL returning "" (see D10022).
-- Add two positive testcases:
--- test the error case using kvm_write on a O_RDONLY descriptor.
--- test the "no error" case using kvm_read(3) and kvm_nlist(3) as
    helper routines and by injecting a bogus error message via
    _kvm_err (an internal API) _kvm_err was used as there isn't a
    formalized way to clear the error output, and because
    kvm_nlist always returns ENOENT with the NULL terminator today.
- kvm_open, kvm_open2:
-- Add some basic negative tests for kvm_open(3) and kvm_open2(3).
   Testing positive cases with a specific
   `corefile`/`execfile`/`resolver` requires more work and would require
   user intervention today in order to reliably test this out.

Reviewed by:	markj
MFC after:	2 months
Sponsored by:	Dell EMC Isilon
Differential Revision:	D10024