fix(tmt): Update variable after becoming a Symbol

The attribute name was renamed to sym_name in the dialect. This change aligns with this rename.

Variable must now also exist in the module scope, as opposed to the function scope and have at least one use to be serialized/ deserialized correctly. Therefore, variable_read has been added to each test case and the explicit variable_read test has been removed, since it's now duplicate.

Signed-off-by: Luke Hutton luke.hutton@arm.com