test-cutils: Test integral qemu_strto* value on failures

test-cutils: Test integral qemu_strto* value on failures

We are inconsistent on the contents of *value after a strto* parse
failure.  I found the following behaviors:

- parse_uint() and parse_uint_full(), which document that *value is
  slammed to 0 on all EINVAL failures and 0 or UINT_MAX on ERANGE
  failures, and has unit tests for that (note that parse_uint requires
  non-NULL endptr, and does not fail with EINVAL for trailing junk)

- qemu_strtosz(), which leaves *value untouched on all failures (both
  EINVAL and ERANGE), and has unit tests but not documentation for
  that

- qemu_strtoi() and other integral friends, which document *value on
  ERANGE failures but is unspecified on EINVAL (other than implicitly
  by comparison to libc strto*); there, *value is untouched for NULL
  string, slammed to 0 on no conversion, and left at the prefix value
  on NULL endptr; unit tests do not consistently check the value

- qemu_strtod(), which documents *value on ERANGE failures but is
  unspecified on EINVAL; there, *value is untouched for NULL string,
  slammed to 0.0 for no conversion, and left at the prefix value on
  NULL endptr; there are no unit tests (other than indirectly through
  qemu_strtosz)

- qemu_strtod_finite(), which documents *value on ERANGE failures but
  is unspecified on EINVAL; there, *value is left at the prefix for
  'inf' or 'nan' and untouched in all other cases; there are no unit
  tests (other than indirectly through qemu_strtosz)

Upcoming patches will change behaviors for consistency, but it's best
to first have more unit test coverage to see the impact of those
changes.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230522190441.64278-4-eblake@redhat.com>