With 3adbbfb5 (lint: Add linter for testhelper.Run, 2023-08-24) we
added a linter to detect if a package did not contain a `TestMain`
function. This was limited to checking non-test packages as each
analysis pass is self-contained and does not allow us to
cross-reference another package.

To allow us to reliably detect when a primary package does not have
tests, but its `_test` package does, start exporting a
`testhelperFact` that indicates if a package has a `TestMain` function.
When analyzing a `_test` package, we can import the fact of its related
package and prevent false positives.