Enable formatting in CI

Enable formatting in CI by doing:

- Add a section to the `test.sh` scripts to run the formatter (guarded by
  the env variable `DO_FMT`) for all crates (bitcoin, hashes, internals).
- Add `DO_FMT` to the nightly `Tests` CI job.

.github/workflows/rust.yml

@@ -53,6 +53,7 @@ jobs:
         uses: dtolnay/rust-toolchain@nightly
       - name: Running test script
         env:
+          DO_FMT: true
           DO_BENCH: true
           AS_DEPENDENCY: false
           DO_NO_STD: true

bitcoin/contrib/test.sh

@@ -111,6 +111,16 @@ then
     )
 fi
 
+# Run formatter if told to.
+if [ "$DO_FMT" = true ]; then
+    if [ "$NIGHTLY" = false ]; then
+        echo "DO_FMT requires a nightly toolchain (consider using RUSTUP_TOOLCHAIN)"
+        exit 1
+    fi
+    rustup component add rustfmt
+    cargo fmt --check
+fi
+
 # Bench if told to, only works with non-stable toolchain (nightly, beta).
 if [ "$DO_BENCH" = true ]
 then

hashes/contrib/test.sh

@@ -89,6 +89,16 @@ if [ "$DO_ASAN" = true ]; then
     cargo test --lib --no-default-features --features="$FEATURES" -Zbuild-std --target x86_64-unknown-linux-gnu
 fi
 
+# Run formatter if told to.
+if [ "$DO_FMT" = true ]; then
+    if [ "$NIGHTLY" = false ]; then
+        echo "DO_FMT requires a nightly toolchain (consider using RUSTUP_TOOLCHAIN)"
+        exit 1
+    fi
+    rustup component add rustfmt
+    cargo fmt --check
+fi
+
 # Bench if told to, only works with non-stable toolchain (nightly, beta).
 if [ "$DO_BENCH" = true ]
 then

internals/contrib/test.sh

@@ -52,3 +52,13 @@ fi
 if [ "$DO_DOCS" = true ]; then
     RUSTDOCFLAGS="-D warnings" cargo +stable doc --all-features
 fi
+
+# Run formatter if told to.
+if [ "$DO_FMT" = true ]; then
+    if [ "$NIGHTLY" = false ]; then
+        echo "DO_FMT requires a nightly toolchain (consider using RUSTUP_TOOLCHAIN)"
+        exit 1
+    fi
+    rustup component add rustfmt
+    cargo fmt --check
+fi