Add dependency checking on test initialization

[tedmasterweb]

This is a feature request to add some kind of test initialization in which, if certain system dependencies aren't met, the test is skipped with the reason why being printed to the screen.

For example, the gzip test requires both exec() not be disabled and that curl be installed and accessible on the command line. The idea is that the base class has some kind of initialization routine that sets an internal variable and runtest() only executes if the initialization flag is green. If the flag is red, the cause of the error is printed so that the end user knows what's failing and why.