Clarify the VK_LOADER_DRIVERS_SELECT example

Clarify the VK_LOADER_DRIVERS_SELECT example

In the documentation examples, if we set
`VK_LOADER_DRIVERS_SELECT=nvidia`, we are saying to the loader to select
only the driver whose JSON manifest is called exactly `nvidia`.

This is likely not what we want, because the Nvidia drivers are usually
called something like `nvidia_icd.json`, and not just `nvidia` without a
file extension.

In the "Behavior" column it already states that:
"Since drivers don’t have a name like layers, this glob is used to
compare against the manifest filename."
So, to avoid confusion, we just need to change the example to reflect
how the loader glob works.

This commit also adds an additional automated test to ensure that this
full-name string glob actually works as described in the documentation.

Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>