Cut down on CI steps?

[madsmtm]

Since #1344 and #1384, we have a lot of CI steps.

We should maybe consider cutting down on it, by:

• Only testing --release and --features parallel on a few select platforms?
• Merging some tests that run on the same platform (related to Better CI for cross-compiling #1390).

Because while they may run in parallel, they still consume both power, and from my understanding also money from the Rust org (?).

[briansmith]

The tests added in PR #1384 are important for preventing regressions and more tests along those lines are needed. In the last year or so there have been lots of regressions in cc-rs that could be prevented with such testing.

[madsmtm]

I didn't mean to imply that we should cut down on test coverage; I only meant that I fear that the way our current CI runs may be overly wasteful, and that in turn discourages me from adding more tests.