Speed up the schema definition tests in CI. (#146)

### What

The OpenAPI schema definition tests can take several minutes to run, for
two reasons:

1. The tests call `cargo run`, which can trigger recompilation of all
dependencies if they haven't been built for this specific target before.
2. On CI, the tests were running in "debug" mode, which means they
couldn't make use of the shared build cache, and were taking a few
minutes as opposed to, well, less time than that.

### How

I have made sure the tests run in release mode on CI (and added `cargo
nextest` so things are consistent with the other test jobs).

I rewrote the schema definition generator to expose a helper function
which can be called directly by tests, rather than using `cargo` to
rebuild and rerun the program.

I also removed the Rust cache used by the formatting job because it
doesn't actually compile code, which can mean that it ends up writing an
empty cache, forcing all other jobs to recompile.

.github/workflows/check-format.yaml

@@ -17,10 +17,6 @@ jobs:
         run: |
           rustup show
 
-      - uses: Swatinem/rust-cache@v2
-        with:
-          shared-key: "build" # share the cache across jobs
-
       - name: check formatting
         run: |
           cargo fmt --all --check

.github/workflows/schema-definitions.yaml

@@ -13,11 +13,14 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: install tools
-        run: rustup show
+        run: |
+          rustup show
+          # install cargo-nextest
+          curl -LsSf https://get.nexte.st/latest/linux | tar zxf - -C ${CARGO_HOME:-~/.cargo}/bin
 
       - uses: Swatinem/rust-cache@v2
         with:
           shared-key: "build" # share the cache across jobs
 
       - name: OpenAPI Definitions
-        run: cargo test --bin openapi-generator
+        run: cargo nextest run --no-fail-fast --release --bin openapi-generator

crates/documentation/openapi/src/main.rs

@@ -1,31 +1,30 @@
+use std::io;
+
 use ndc_postgres::configuration::RawConfiguration;
 use schemars::{gen::SchemaSettings, schema::RootSchema};
 
-fn main() {
-    let schema: RootSchema = SchemaSettings::openapi3()
-        .into_generator()
-        .into_root_schema_for::<RawConfiguration>();
+fn main() -> io::Result<()> {
+    let schema = generate_schema();
+    serde_json::to_writer_pretty(io::stdout(), &schema)?;
+    Ok(())
+}
 
-    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+fn generate_schema() -> RootSchema {
+    SchemaSettings::openapi3()
+        .into_generator()
+        .into_root_schema_for::<RawConfiguration>()
 }
 
 #[cfg(test)]
 mod tests {
-    use std::process::Command;
+    use super::*;
 
     #[test]
-    fn main() {
-        let command_output = Command::new("cargo")
-            .arg("run")
-            .arg("--bin")
-            .arg("openapi-generator")
-            .output()
-            .expect("Failed to run schema generation")
-            .stdout;
-
-        let generated_schema: String =
-            String::from_utf8(command_output).expect("Failed to read snapshot");
+    fn main() -> io::Result<()> {
+        let generated_schema = generate_schema();
+        let generated_schema_json = serde_json::to_string_pretty(&generated_schema)?;
 
-        insta::assert_snapshot!(generated_schema);
+        insta::assert_snapshot!(generated_schema_json);
+        Ok(())
     }
 }