Add SimpleCov to collect code coverage info #369

Job	Run time
test (ubuntu-24.04, 3.1, gemfiles/rails_6.1.gemfile)	22s
test (ubuntu-24.04, 3.1, gemfiles/rails_7.0.gemfile)	22s
test (ubuntu-24.04, 3.1, gemfiles/rails_7.1.gemfile)	26s
test (ubuntu-24.04, 3.1, gemfiles/rails_7.2.gemfile)	25s
test (ubuntu-24.04, 3.2, gemfiles/rails_6.1.gemfile)	18s
test (ubuntu-24.04, 3.2, gemfiles/rails_7.0.gemfile)	22s
test (ubuntu-24.04, 3.2, gemfiles/rails_7.1.gemfile)	24s
test (ubuntu-24.04, 3.2, gemfiles/rails_7.2.gemfile)	28s
test (ubuntu-24.04, 3.2, gemfiles/rails_8.0.gemfile)	29s
test (ubuntu-24.04, 3.3, gemfiles/rails_6.1.gemfile)	19s
test (ubuntu-24.04, 3.3, gemfiles/rails_7.0.gemfile)	20s
test (ubuntu-24.04, 3.3, gemfiles/rails_7.1.gemfile)	24s
test (ubuntu-24.04, 3.3, gemfiles/rails_7.2.gemfile)	25s
test (ubuntu-24.04, 3.3, gemfiles/rails_8.0.gemfile)	23s
	5m 27s

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