`x.py test --no-doc` enables testing benchmarks

[ehuss]

./x.py test --stage=0 --no-doc src/libcore will also test the benchmarks in libcore.
./x.py test --stage=0 src/libcore does not test the benchmarks.

It is a little surprising that --no-doc would enable benchmarks. This block of code is responsible for enabling benchmarks.

The bigger question is, should benchmarks always be tested? There is a problem that benchmarks are currently not tested on CI, so when they break nobody knows about it (see #54176 and #61816). It could maybe be done with something like --all-targets, and then --no-doc would stay the same. At least with libcore, it only adds a few seconds to build and run (benchmarks only run one iteration in test mode).