[Formulus] Replace binary ZIP test fixtures with a script that generates them at test time

[najuna-brian]

Description:

The files synkronus/testdata/bundles/valid_bundle01.zip and valid_bundle02.zip were removed from the repository in #572 because the F-Droid scanner flagged committed binary files during the app store submission process.

These fixtures were used by TestBundleChanges_FieldAddition and TestPushBundleGeneratesAppInfo to test bundle pushing, version switching, and app info generation against realistic bundle structures.

What needs to be done:

Replace the committed ZIP files with a test helper script or TestMain setup that programmatically generates the required bundle ZIPs at test time, so:

• valid_bundle01.zip - a bundle with an example form containing 7 fields
• valid_bundle02.zip - a bundle with an example form containing 8 fields (adds a lastname field), and includes app/THIS_IS_VERSION_2.txt with content "Something..."

Once the generator exists, restore TestBundleChanges_FieldAddition and TestPushBundleGeneratesAppInfo using the generated bundles instead of opening files from disk.

Acceptance criteria:

• No binary files committed to the repo
• All previously removed test cases pass again
• Bundle generation happens automatically as part of the test setup