Reduce CI time

[taiki-e]

The slowest job in our CI (build (nightly)) takes over an hour. This is a really annoying situation, so let's investigate if we can shorten the time.

[taiki-e]

For now, my goal with this issue is to complete all jobs within 30 minutes. The following jobs currently violate this.

• build (nightly) -- 1h 0m 49m
• build (nightly-2020-12-26) -- 34m 41s see Reduce CI time #55 (comment)
• san (address) -- 37m 46s see Reduce CI time #55 (comment)
• san (memory) -- 32m 21s see Reduce CI time #55 (comment)

The only solution for san (..) may be to reduce the number of iterations in tests. see #55 (comment)
We are placing our integration tests in src/tests, so we will not be able to reduce the number of test binaries any further.

As for build (..), it is build-only, so I guess basically we need to reduce the number of builds.

[taiki-e]

As for san (..), testing in the debug build was taking more than twice as long as testing in the release build, mostly from doctests (note: test binaries will be created for each doctest), so I disabled doctests in san+debug builds.

• san (address) -- 37m -> 21m
• san (memory) -- 32m -> 16m
• san (thread) -- 24m -> 17m

[taiki-e]

As for build (..), aa210ee has improved the situation, but build (nightly) is still slow.

• build (nightly) -- 1h -> 49m
• build (nightly-2020-12-26) -- 34m -> 27m