fix: test: fix macOS build for plugin unit test

MR !10753 breaks macOS build for plugin unit test as its linker doesn't
supports `--wrap` option, which is used in in order to mock the function
`isc_file_exits()`.

To work around the problem, a mocked `isc_file_exits()` is implemented
inside the plugin test as a static function before inlining the file
using it, which effectively links to this version rather than the isclib
one.

Closes #5455

Merge branch '5455-fix-macos-build-plugintest' into 'main'

See merge request isc-projects/bind9!10796