Introduce a tester for extfs helpers

[mooffie]

Here's a tester intended to test the "list" command of extfs helpers.

(Others would use the phrase "test framework" instead of "tester". I'm trying to sound friendly.)

I provide with it tests for uzip, urar, uzoo, and lslR. That's to demonstrate the power of the tester. If this tester gets merged into MC I'll provide tests for more helpers (anybody could, though; it's very easy).

Why

The dire problem of us not currently having tests for covering extfs has been mentioned several times here.

How it works

The tester works by feeding the helpers example input, parsing their output and comparing it to the known correct output.

To understand this better, I ​uploaded the tester here for easier browsing; specifically:

• See the ​README (html format).
• See the ​data directory. The purpose of the '.input' and '.output' files should be self evident even if you don't read the docs.
• As you'll shortly see, each helper needs a very minor modification to make it possible to feed it "fake" input.

How it looks like

(a) Either do "make check"; or:
(b) Run the tester directly, with "run", to see colorful output:

(b1) ...when everything works alright:

(b2) ...when some helper has a bug:

[mooffie]

Some random notes:

1. Yeah, I know the masses are ​clamoring for FISH tests. At the moment I'm not too familiar with the FISH scripts and I want to leave that out of this ticket.

2. A few words about "integration tests" vs "unit tests": There's ​another method to do testing, if MC had scripting support. That method lets us discover many bugs in the VFS, even bugs we didn't intend to cover. But, as Yury ​explains there, those are "integration tests", and as such they have a drawback: they don't give us the direct control "unit tests" give us on the input each component is fed. I.e., in our case we need to feed a helper various inputs, not just the input our system provides, and unit tests are better for this.

[mooffie]

(Since this ticket includes a test for uzoo, it's dependent on #3696.)

[mooffie]

(In #3729) It will be very easy to fix these once we have #3730 in.