Add "unit" tests that test against parsed code

Implements Issue #22.

Under `tests/Psecio/Parse`, implemented `ParseTest.php`,
`ParseTestVisitor.php` and `Tests/TestSessionRegenFalse_ParseTest.php` (yeah,
crazy name there. May need to figure out a better naming convention).

* `ParseTest.php` implements an abstract base class for all the parsing-based
  tests.
* `ParseTestVisitor.php` implements a simple parse tree visitor for use in
  testing.
* `Tests/TestSessionRegenFalse_ParseTest.php` implements several parse string
  tests for `TestSessionRegenFalse`, including one for the problem I found and
  fixed in PR #15.
* `Tests/TestAvoidGlobalsUse_ParseTest.php` implements parse string tests for
  `TestAvoidGlobalsUse`.

Implementing a test is as simple as creating a test case that extends
`\tests\Psecio\Parse\ParseTest`. It must implement `parseSampleProvider()` and
`buildTest()`. The `buildTest()` method should return an instance of the unit
under test (e.g. `TestSessionRegenFalse`). Then `parseSampleProvider()` should
return a list of PHP code string (with initial `<?php` omitted) and result
pairs. Each PHP code string will be parsed, with the results evaluated against
the results of `buildTest()` via the `PhpParser\NodeTraverser`. If the results
match the result value, the test passes. Otherwise, it fails.