Create asserts test variables being set, unset more easily

[zambrovski]

Scenario

As a developer of process (model) test, I'm interested in verifying that the process is orchestrating correctly and sets the process variables as expected.

Current Behaviour

None.

Wanted Behaviour

I want to be be able to assert that variables are set correctly. The assertions should work easily with camunda-bpm-assert.

[mmiikkkkaa]

Sounds like the changes I added within #80

[zambrovski]

In camunda-bpm-assert you can use asserts for the variables:

• https://github.com/camunda/camunda-bpm-assert/blob/master/docs/User_Guide_BPMN.md#instance-hasvariables
• https://github.com/camunda/camunda-bpm-assert/blob/master/docs/User_Guide_BPMN.md#instance-hasnovariables

This concept can be extended to checking for:

• existence of global variable
• existence of local variable
• existence of historic variable
• value check of global variable
• value check of local variable
• value check of historic variable

Something similar has been already done in Camunda BPM JGiven:

https://github.com/holunda-io/camunda-bpm-jgiven/blob/696dd6734bb13605115967ab4667fceb5996421b/extension/src/main/kotlin/io/holunda/camunda/bpm/extension/jgiven/ProcessStage.kt#L236