Added an example for linear regression input test

Summary:

* LinregrOutputTestCase:
  - Answer file is a single file containing all R results
  - Result file name contains corresponding parameter values
  - Answers are created by external program (here we use R)

* LinregrInputTestCase
  - Multiple answer files, each of which is for one test case
  - Result & answer file names do not have parameter values, but instead
  use an integer to index the files
  - Answers are created by a run of the test itself. And the output is
  used as the baseline for future tests.