state: add more tests for block validation (#3674)

* Expose priv validators for use in testing

* Generalize block header validation test past height 1

* Remove ineffectual assignment

* Remove redundant SaveState call

* Reorder comment for clarity

* Use the block executor ApplyBlock function instead of implementing a stripped-down version of it

* Remove commented-out code

* Remove unnecessary test

The required tests already appear to be implemented (implicitly) through
the TestValidateBlockHeader test.

* Allow for catching of specific error types during TestValidateBlockCommit

* Make return error testable

* Clean up and add TestValidateBlockCommit code

* Fix formatting

* Extract function to create a new mock test app

* Update comment for clarity

* Fix comment

* Add skeleton code for evidence-related test

* Allow for addressing priv val by address

* Generalize test beyond a single validator

* Generalize TestValidateBlockEvidence past first height

* Reorder code to clearly separate tests and utility code

* Use a common constant for stop height for testing in state/validation_test.go

* Refactor errors to resemble existing conventions

* Fix formatting

* Extract common helper functions

Having the tests littered with helper functions makes them less easily
readable imho, so I've pulled them out into a separate file. This also
makes it easier to see what helper functions are available during
testing, so we minimize the chance of duplication when writing new
tests.

* Remove unused parameter

* Remove unused parameters

* Add field keys

* Remove unused height constant

* Fix typo

* Fix incorrect return error

* Add field keys

* Use separate package for tests

This refactors all of the state package's tests into a state_test
package, so as to keep any usage of the state package's internal methods
explicit.

Any internal methods/constants used by tests are now explicitly exported
in state/export_test.go

* Refactor: extract helper function to make, validate, execute and commit a block

* Rename state function to makeState

* Remove redundant constant for number of validators

* Refactor mock evidence registration into TestMain

* Remove extraneous nVals variable

* Replace function-level TODOs with file-level TODO and explanation

* Remove extraneous comment

* Fix linting issues brought up by GolangCI (pulled in from latest merge from develop)

state/services.go

@@ -43,7 +43,7 @@ type EvidencePool interface {
 	IsCommitted(types.Evidence) bool
 }
 
-// MockEvidencePool is an empty implementation of a Mempool, useful for testing.
+// MockEvidencePool is an empty implementation of EvidencePool, useful for testing.
 type MockEvidencePool struct{}
 
 func (m MockEvidencePool) PendingEvidence(int64) []types.Evidence { return nil }