Ignore SQLite3 database files generated by parallel testing

[yahonda]

Summary

Rails 6.0 introduces parallel testing and the default degree of parallelism is configured based on the number of CPU. refer #34735 #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

$ grep processor /proc/cpuinfo | wc -l
6
$ 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)
$