Trial's TestLoader should sort tests in a specfic order

[jml]

It should clearly and cleanly provide:

• Sorting alphabetically
• Sorting reverse alphabetically
• Sorting randomly

The sorting code should be basically separate from the loading code.

[therve]

#2559 was a duplicate of this.

[warner]

I'd like to add "Sorting according to argv[]" to the list of options. I have frequently run into tricky failures that are uncovered while --random was enabled. In trying to isolate which pair of tests had the problem, I want to use 'trial TEST1 TEST2 TEST3', and remove tests one at a time. But without a way to preserve the order that I specify via sys.argv, I have no way to run a set of tests in the same order as the --random pass chose.

Note that --random=origseed won't help, because on later runs it is shuffling an entirely different list of test cases. Now, if we used --random to seed a consistent hashing -based permutation, this might Just Work, but I think letting people use sys.argv in the obvious way would be less surprising.

[Screwtape]

#3088 (and hence its dupe #3249) were duplicates of this.

Resummarizing to hopefully make this bug easier to find (old title was "TestLoader's sorting is a mess")

[warner]

basic patch to leave test cases in argv order

[warner]

that patch I just attached has the following drawbacks:

• requires O(n²) time to avoid duplicates (but really, how many test-case arguments do you intend to add?)
• no tests yet

[exarkun]

I also frequently want the argv ordering feature.

[exarkun]

This ticket is perhaps now obsolete?

• #5520 implemented running in command line order.
• #5787 proposes implementing a --order option supporting arguments such as "toptobottom". "alphabetical" would fit in with this.