After running `bundle exec rake test:sqlite3` and `bundle exec rake test:sqlite3_mem`
on my VM I noticed that it had created untracked files:
```bash
vagrant@ubuntu-bionic:/rails/activerecord$ git status
Untracked files:
(use "git add <file>..." to include in what will be committed)
db/
file::memory:
```
To prevent them from being accidentally committed I put 'file::memory:' to
`activerecord/db/` folder and added the folder to .gitignore
Also, we could consider fixing this by removing `db/` folder in each test that
creates the folder.
It would be great if someone confirms that it happens not only on my VM.
* Global ignores at toplevel .gitignore
* Component-specific ignores in each toplevel directory
* Remove `actionview/test/tmp/.keep` for JRuby
```
rm actionview/test/tmp/ -fr
cd actionview/
bundle exec jruby -Itest test/template/digestor_test.rb
```
Related to #11743, #30392.
Closes#29978.
