x/build: shard the shootout on builders

[bradfitz]

shard the shootout on builders, moving them into cmd/dist/test tasks

Also, disable on linux-arm and linux-arm-arm5. Maybe just compile only. Or run in some short mode.

They take way too long.

/cc @adg @davecheney

[bradfitz]

Turns out test/bench/shootout/timing.sh can already filter which tests it runs.

case $# in
0)
        run="fasta revcomp nbody binarytree fannkuch regexdna spectralnorm knucleotide mandelbrot meteor pidigits threadring chameneos"
        ;;
*)
        run=$*
esac

So test.go just needs to find that line and create sub-tests for each.

But later it'd be nice to rewrite that shell in Go, so it runs in more places.

And add a -fast flag to make it do fewer iterations (like 10 instead of 5000000), just to test coverage for the builders.

[bradfitz]

Note: now that #12668 is fixed, the coordinator should un-blacklist the "shootout" tests for linux-arm.

[gopherbot]

CL https://golang.org/cl/14738 mentions this issue.