Opening in favor of #226

This is a:

• bug fix PR with no breaking changes — please ensure the base branch is master
• new functionality — please ensure the base branch is the latest dev/ branch
• a breaking change — please ensure the base branch is the latest dev/ branch

Description & motivation

From #226:

I love the expression_is_true schema test, but often it feels like the tests I use them for should be a child of the column I'm testing against, instead of the whole relationship. So I modified the expression_is_true test to do exactly that.

Left the original tests as is (no regressions), and added 2 more tests to test with condition set and without it.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md