gitutils: add diffStrings, diffFiles, and use it in testament to compare expected vs gotten

[timotheecour]

• add diffStrings, diffFiles (can be customized in future with options if needed)
• replaces all instances of diff -uNdr with an API call
• testament now uses diffStrings to compare expected vs gotten, making it easier to see what differs (in particular for whitespace issues, but not limited to that)
• expose tempfiles.genTempPath

example

I added (and reverted) a temporary commit that added intentional changes in test "expected output", to show what it looks like in CI, see https://gist.github.com/timotheecour/515dde92cb29ce359a3f4b9546c247fd

note: unrelated, preexisting bug in std/tempfiles

as shown in https://gist.github.com/timotheecour/515dde92cb29ce359a3f4b9546c247fd, it looks like randomPathName is buggy on windows: see #17898;
this doesn't affect this PR though because I'm using a prefix diffStrings_a and diffStrings_b so there is no clash here (but it's still a bug that should be fixed in future work)

[Araq]

We could revive experimental/diff.nim for this.

[timotheecour]

PTAL

• added runnableExamples to experimental/diff and cross-linked both modules
• added notes in PR description showing how it looks like in CI when there's a diff (via a fake commit which inserted spec diffs which i've reverted)
• identified a pre-existing bug (randomPathName called twice in a row can return the same string on windows #17898) which doesn't affect this PR

We could revive experimental/diff.nim for this.

this can be done in future work and would be a 2 line fix once experimental/diff gains the required features for this:

• it doesn't have a human readable output yet, we could in future add toPretty to render it.
• note also that git diff has more options (eg --word-diff, --color-moved etc) and honors a users' git config to customize how it's rendered (eg with colors etc)

Currently, a primary use case for experimental/diff over git diff is when shelling out is not an option (eg because it needs to be called at high frequency etc). This is not the case here.

[timotheecour]

showing expected is still useful (even with showing the diff) so users can copy paste it
(and showing the given is also useful for understanding the output)