Add tests for plugins without tests

We should test the plugins in this repository for what I hope are obvious reasons.

Plugins missing (integration) tests

Elements

• collect_integration - !35 (merged)
• collect_manifest
• distutils - !38 (merged)
• fastboot_bootimg
• fastboot_ext4
• flatpak_image
• flatpak_repo
• makemaker - !40 (merged)
• meson - !36 (merged)
• modulebuild - !41 (merged)
• oci
• qmake
• x86image

Sources

• git_tag
• cargo

Sources missing standardised source tests

• git_tag - !34 (merged)
• cargo

Other things to do

• Add any tests that were missed in the migration of plugins from bst-core - !34 (merged)
• Cleanup the test suite a bit - !34 (merged)