make zig build test check for conforming formatting

There was already `zig build test-fmt` but now `zig build test` depends
on that one.

The CI scripts no longer need explicit logic since they already do
`zig build test`.

ci/x86_64-linux-debug.sh

@@ -57,13 +57,6 @@ unset CXX
 
 ninja install
 
-# TODO: move this to a build.zig step (check-fmt)
-echo "Looking for non-conforming code formatting..."
-stage3-debug/bin/zig fmt --check .. \
-  --exclude ../test/cases/ \
-  --exclude ../doc/ \
-  --exclude ../build-debug
-
 # simultaneously test building self-hosted without LLVM and with 32-bit arm
 stage3-debug/bin/zig build \
   -Dtarget=arm-linux-musleabihf \