Backport tests: Allocate gvariant data from the heap to guarantee alignment to glib-2-56

On glib-2-58 branch we don't have !455 (merged), thus we need aligned data for the gvariant tests to not fail on i686.

Fixes #1626 (closed)

Backport of !552 (merged) to glib-2-56. Note that I don’t plan on making another glib-2-56 release for this; it’s just so that downstream maintainers can share the patch if they need.