[roll] Roll fuchsia [zircon][test] Add missing members to maybe-standalone lib.

The maybe-standalone-test library exists to make it easy for tests
which depend on functionality provided by the standalone-test library
to be built for multiple environments, but bailout of the test early
if the standalone-test functionality is not present in the environment
which the test is running in.

Most of the maybe-standalone-test library simply declares weak symbols
for the standalone-test library methods, and then tests at runtime to
see if the symbols are available.  If so, the maybe-library simply
passes the call along to the not-maybe-library.  If not, the
maybe-library typically just returns something like INVALID_HANDLE for
the resource which the user is requesting.

Over time, new methods have been added to the standalone library, but
did not have corresponding thunks added to the maybe version of the
library.  Go ahead and continue the pattern by adding thunks for the
following standalone-test library methods:

+ `zx::unowned_resource GetIoportResource()`
+ `zx::unowned_resource GetIrqResource()`
+ `zx::unowned_vmo GetVmo(std::string_view name)`
+ `zx::unowned_channel GetNsDir(std::string_view name)`

`GetVmo` in particular is used in a subsequent CL to implement a
regression test in core/time.

Original-Original-Bug: 515184484
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1627696
Original-Original-Revision: 4b29dac2bc0c8e4eaeb42be3115174fa672a62d3
GitOrigin-RevId: f764435c9b82840657fbc333e2d185e630021bdc
Change-Id: I758a3f102c67843b4daf49831a1278a464ec968f