Add postprocessor tests

[dschwen]

Refs #6

[moosebuild]

Job Documentation, step Sync to remote on d9a5114 wanted to post the following:

View the site here

This comment will be updated on new commits.

[moosebuild]

Job Coverage, step Generate coverage on d9a5114 wanted to post the following:

Coverage

	d06b12	#7 d9a511
	Total	Total	+/-	New
Rate	25.05%	27.72%	+2.67%	100.00%
Hits	688	763	+75	7
Misses	2059	1990	-69	0

Diff coverage report

Full coverage report

This comment will be updated on new commits.

[moosebuild]

Job Precheck, step Clang format on 18de4ce wanted to post the following:

Your code requires style changes.

A patch was auto generated and copied here
You can directly apply the patch by running, in the top level of your repository:

curl -s https://mooseframework.inl.gov/swift/docs/PRs/7/clang_format/style.patch | git apply -v

Alternatively, with your repository up to date and in the top level of your repository:

git clang-format d06b12781ca9e58d41de4cccaf4c7d93fa5ca22a