Fix minitest expectation deprecation warnings

[aidanharan]

Fix the minitest expectation deprecation warnings from the coerced tests.

Example from https://travis-ci.org/rails-sqlserver/activerecord-sqlserver-adapter/jobs/654830621?utm_medium=notification&utm_source=github_status

DEPRECATED: global use of must_equal from activerecord-sqlserver-adapter/test/cases/coerced_tests.rb:618. Use _(obj).must_equal instead. This will fail in Minitest 6.

DEPRECATED: global use of must_equal from activerecord-sqlserver-adapter/test/cases/coerced_tests.rb:619. Use _(obj).must_equal instead. This will fail in Minitest 6.
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This is a follow-on from PR #718