Replace NodeJS dict aliases with maps and named field reads

src/testing/index.ts

@@ -1,5 +1,5 @@
-// Test support: the current user's IDs, a fresh temporary directory, and a
-// plain command runner. It imports no source module, so the repository-level
+// Test support: the current user's IDs, a fresh temporary directory, the
+// inherited environment, and a plain command runner. It imports no source module, so the repository-level
 // tests in tests/, which reach src/ only through `#testing`, never touch
 // internals.
 import { spawn } from "node:child_process";
@@ -33,14 +33,26 @@ export interface CommandResult {
   stderr: string;
 }
 
+// This process's environment as a map from variable name to value.
+export function inheritedEnvironment(): Map<string, string> {
+  const result: Map<string, string> = new Map();
+  for (const [name, value] of Object.entries(process.env)) {
+    if (value !== undefined) result.set(name, value);
+  }
+  return result;
+}
+
 // Runs a command with stdin closed and only the given environment, and
 // collects its stderr.
 export async function runCommand(
   command: string,
   args: string[],
-  env: NodeJS.ProcessEnv,
+  env: ReadonlyMap<string, string>,
 ): Promise<CommandResult> {
-  const child = spawn(command, args, { env, stdio: ["ignore", "ignore", "pipe"] });
+  const child = spawn(command, args, {
+    env: Object.fromEntries(env),
+    stdio: ["ignore", "ignore", "pipe"],
+  });
   const [stderr, closed] = await Promise.all([text(child.stderr), once(child, "close")]);
   const exitCode: unknown = closed[0];
   return { success: exitCode === 0, stderr };