Reuses standard file upload storage to cover Terraform
states backup

Adds a seed for generating terraform state files
under each project

Manual testing (in local GDK env):

- Verfied seed addition via `bin/rake db:seed_fu FILTER=terraform`
- Verified seed addition is repeatable, with no-ops if versions
  pre-exist
- Created backup: `bin/rake gitlab:backup:create`
- Verified output shows up:

```
2021-06-05 23:48:54 +0530 -- Dumping terraform states ...
2021-06-05 23:48:54 +0530 -- done
```

- Verified tar contains terraform_state.tar.gz
- Verified terraform_state.tar.gz carries content
- Destroyed all records: `Terraform::State.destroy_all`
- Deleted all files: `rm -rf shared/terraform_state/*`
- Performed restore: `bin/rake gitlab:backup:restore`
- Verified `Terraform::State.all` shows back older records
- Verified association of builds to some of these records
- Verified `shared/terraform_state/` is repopulated
- Verified `Terraform::StateVersion.each do |sv| pp sv.file.read end`
  works and shows the seed-added data
- Verified skipping of `terraform_state` keyword works:

```
2021-06-06 00:11:33 +0530 -- Dumping terraform states ...
2021-06-06 00:11:33 +0530 -- [SKIPPED]
```

Verified with same workflow as above (backup, destroy, delete,
then restore) that skipped backup variant does not contain
`terraform_state.tar.gz` and does not show up those files after
restore

Miscellany:

- Adds a missing test for LFS backups
- Adds File operation fixes to neighboring tests
  so when they fail RSpec is able to show diffs
- Modified test validates unknown SKIP values not
  breaking backups
  - This is useful to catch any regressions during
    upgrades where pre-installs scripts may refer
    to a new SKIP value that isn't recognized by
    the old version

Changelog: added