Document a difference between pluck and select [ci skip]

Explain that `pluck` differs from `select` in that it does not construct `ActiveRecord` objects and thus model-level overrides are unavailable.
Add a test case for exists? with multiple values
Fix method name typos
Merge pull request #12130 from egilburg/patch-1

Document a difference between pluck and select [ci skip]
Merge pull request #12139 from vipulnsward/typos_av

Fix method name typos
fix actionview and activemodel test cases typos
Merge pull request #12143 from rajcybage/fixing_typos

fix actionview and activemodel test cases typos
push constant lookup in the a cache object
only construct the FixtureSet with AR models
push logic outside the FixtureSet constructor
no need to dup
Fix typo in set_callback docs. [ci skip]
Merge pull request #12148 from gzohari/callback-typo

Fix typo in set_callback docs. [ci skip]
Do not use instance variables if they are not reused elsewhere
Fix failure with minitest 5.0.7

Require minitest/mock in test where stub method is needed.

Apparently this was being "imported" by minitest pride plugin, which
previously required "minitest/autorun", that required "minitest/mock",
making the method available by chance.

It has been changed in minitest:
https://github.com/seattlerb/minitest/commit/595ce955c0d89575726d10fc18d2afd8f334bcbe

So we need to make sure we require what we really need where necessary.
ensure @model_class is initialized
don't depend on object state when reading fixture files
auto link hm:t fixture join tables like habtm
make the duplicate code more similar
reduce duplication somewhat