[build-script] Skip unsupported 'check-swift-only_long-optimize' test variant

[rintaro]

What's in this pull request?

build-script --long-test --test-optimized

This tries to run check-swift-only_long and check-swift-only_long-optimize
But check-swift-only_long-optimize does not exist.

--- check-swift-only_long-optimize-linux-x86_64 ---
ninja: error: unknown target 'check-swift-only_long-optimize-linux-x86_64'
-- check-swift-only_long-optimize-linux-x86_64 finished --

CC: @gribozavr
Why it's not supported?

---

Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

[gribozavr]

@rintaro Sorry for the delay, finally got to reconstruct the reasoning behind this limitation.

We would like to be able to mark tests both as executable and long, and then have check-swift-only_long-optimize run them in -O mode. The reason why we can't currently is that lit's limit_to_features variable only allows us to specify a union of desired features. That is, if we specify limit_to_features = ['executable_test', 'long_test'] lit will run all executable and all long tests (even non-executable), which will waste time.

I'd like to fix this, but I'm not sure what would be the best way. One hacky way would be to add a non_executable_test feature. Every long test would have to declare whether it is executable or not.

Another approach would be to have our own discovery plugin for lit, which would be able to implement any filtering that we like, but that's a much bigger time investment.

[gribozavr]

Implemented in #2909.

[rintaro]

Closing in favor of #2909