Issue 18715 - Non-documented unittests should not use unpredictableSeed or default Random alias

[n8sh]

Rationale in review comment by @wilzbach #6410 (comment):

Use a fixed seed and a fixed engine, everything else might lead to random coverage changes.

This PR applies this across Phobos. Exceptions are made for public unittests that may be meant to demonstrate use of unpredictableSeed and/or the default Random.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	18715	minor	Non-documented unittests should not use unpredictableSeed or default Random alias

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#6414"

[wilzbach]

Note: I have already done some derandomization of Phobos myself (see the PRs below) and there were some folks who wanted unpredictableSeed to stay where it's used:

• Derandomize std.algorithm #5202 (<- this one contains the discussion)
• Mark random coverage line in std.concurrency #5622
• Fix random coverage in std.algorithm.sorting #5572

Imho we have only made bad experiences with random data (hello failing CIs), so I am obviously in favor of this.

[wilzbach]

123_456_789U.repeat ?

[wilzbach]

123_456_789U.repeat(624) ?

[wilzbach]

Better move the popFront into a newline, people don't like this short style:

One statement per line.

(from https://dlang.org/dstyle.html#whitespace)