bios: add getValueIndex unit test coverage

Add unit tests for BIOSEnumAttribute::getValueIndex() to
cover both successful value lookups and invalid input
handling.

Verify that valid enumeration values return the expected
indices and that unknown values continue to throw
std::invalid_argument.

Change-Id: If2efb201d60e8bc1e81d250842c3b5dda7c69c05
Signed-off-by: Meghana Vangapandu <meghanav@ami.com>