tests: Test @ expansion when the variable is empty

Yet another corner case to handle in any future parser rewrites.
tests: Comment variable_expansion test logic

It's not obvious why there are differences between the tests when moving
some lines around. Also remove unused variables from the *File tests.
tests: Make the file expansion more rigorous
tests: Add variable expansion test for @ONLY with empty expansions
Merge topic 'variable-expansion-tests' into next

5b7dec82 tests: Add variable expansion test for @ONLY with empty expansions
58326dd2 tests: Make the file expansion more rigorous
23872099 tests: Comment variable_expansion test logic
fb1997fe tests: Test @ expansion when the variable is empty