[Bounds-Safety] Fix `pragma-attribute-supported-attributes-list.test`

0ec3b972 changed the `counted_by`
attribute to be `LateAttrParseExperimentalExt`. This means the attribute
is no longer supported by `#pragma clang attribute`. However, the
`pragma-attribute-supported-attributes-list.test` wasn't updated
to account for that.

rdar://125400257