[bug fix] Fix unavailable public variables when creating tests

[bartier]

Closes #716

Currently, it's not possible to create public mock variables using the function setVariableName('key', 'value', false) from a TaskMockRunner. This PR attempts to fix this, so it handles the public task variables when VariableInfo[] should be returned by the function getVariables().

Example:

Consider this test idea:

let tmr: tmrm.TaskMockRunner = new tmrm.TaskMockRunner(taskPath);
tmr.setVariableName('FOO', 'BAR', false); // this FOO variable isn't available when we call  getVariables() in the task
tmr.setVariableName('EGG', 'BAR', true);
tmr.run();

If our task needs to access a public variable using getVariables, it only gets the secrets (in this example only EGG).

import tl = require("azure-pipelines-task-lib/task")

async function run() {
  let vars = tl.getVariables()

  console.log(tl.getVariables()) // FOO will not be available here, only EGG will be available
}

run()

I added a test to validate this case, so I hope this PR could be merged as soon as possible =)

[ghost]

All CLA requirements met.

[bartier]

@egor-bryzgalov Could you please have a look at this? I'm not sure if this PR contains the required stuff to be merged :D

[bartier]

@damccorm If there is anything I can clarify in the review process I am willing to help =)

[anatolybolshakov]

Could you please add null check here - so undefined value will be returned for this case, but not an exception being thrown? (the same as for process.env[key])

[anatolybolshakov]

@bartier I'm still not confident about necessity of these changes - why we can't just use TaskMockRunner.setAnswers method to provide mock for GetVariables task-lib method?

[bartier]

@anatolybolshakov Could you please give an example of using setAnswers to achieve that? I'm not sure what field of TaskLibAnswers it would be used.

[anatolybolshakov]

@bartier oh I see - mock-task does not support mocking for getVariables method at the moment and just uses original method.
Probably we could extend mock-task - to be able to set up variables - let us take a look at it