The `rint` tests were commented out when added along with other functions to
the `libc-test` runner, presumably because they were failing. In the meantime,
we moved to using a Zig-implemented `rint` and I found that the tests are now
passing. To celebrate, I'm putting them back into commission.
Having these tests in a working state is nice because functions like `lrint`,
`llrint`, and `nearbyint` depend on `rint` implementation being correct.
Results:
```
$ ./build/stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib
$ stage4/bin/zig build test-libc -Dlibc-test-path=<LIBC-TEST-PATH> -Dtest-filter=rint -fqemu -fwasmtime --summary line
Build Summary: 2209/2209 steps succeeded
```
rpkak