[clang][www] Change 'clang-test' to 'check-clang' on the hacking webpage

This change is https://reviews.llvm.org/D49549 ported to the monorepo layout.

Patch by Arnaud Coomans.

Patch title and message:
```
Change 'clang-test' to 'check-clang' on the hacking webpage

The command to run tests was previously changed from 'clang-test' to 'check-clang'; with 'clang-test' remaining available as a legacy alias (see git-svn-id: https://reviews.llvm.org/rC159483 91177308-0d34-0410-b5e6-96231b3b80d8).

This commit changes the www documentation accordingly.
```