Rails 6.0 introduces parallel testing and the default degree of parallelism is configured based on the number of CPU.
refer https://github.com/rails/rails/pull/34735https://github.com/rails/rails/pull/31900
When any minitest executed under the OS where 2 or more CPU available,
SQLite 3 database files are not git-ignored.
Also updated other files which ignores SQLite database files.
* Steps to reproduce
```
$ git clone https://github.com/yahonda/rep_ignore_sqlite3_databases.git
$ cd rep_ignore_sqlite3_databases/
$ bin/rails test
$ git status
```
* Expected behavior:
- No `Untracked files:`
* Actual behavior:
- SQLite 3 database files appeared
```
$ git status
On branch master
Untracked files:
(use "git add <file>..." to include in what will be committed)
db/test.sqlite3-0
db/test.sqlite3-1
db/test.sqlite3-2
db/test.sqlite3-3
db/test.sqlite3-4
db/test.sqlite3-5
nothing added to commit but untracked files present (use "git add" to track)
$
```
