test: increase coverage for exec() functions

This commit increases code coverage related to the stdout
and stderr outputs of the child_process exec() functions.
Previously, stdout was completely covered, but stderr was
not.

PR-URL: #10919
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-child-process-exec-encoding.js

@@ -0,0 +1,49 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const stdoutData = 'foo';
+const stderrData = 'bar';
+const expectedStdout = `${stdoutData}\n`;
+const expectedStderr = `${stderrData}\n`;
+
+if (process.argv[2] === 'child') {
+  // The following console calls are part of the test.
+  console.log(stdoutData);
+  console.error(stderrData);
+} else {
+  function run(options, callback) {
+    const cmd = `${process.execPath} ${__filename} child`;
+
+    cp.exec(cmd, options, common.mustCall((err, stdout, stderr) => {
+      assert.ifError(err);
+      callback(stdout, stderr);
+    }));
+  }
+
+  // Test default encoding, which should be utf8.
+  run({}, (stdout, stderr) => {
+    assert.strictEqual(typeof stdout, 'string');
+    assert.strictEqual(typeof stderr, 'string');
+    assert.strictEqual(stdout, expectedStdout);
+    assert.strictEqual(stderr, expectedStderr);
+  });
+
+  // Test explicit utf8 encoding.
+  run({ encoding: 'utf8' }, (stdout, stderr) => {
+    assert.strictEqual(typeof stdout, 'string');
+    assert.strictEqual(typeof stderr, 'string');
+    assert.strictEqual(stdout, expectedStdout);
+    assert.strictEqual(stderr, expectedStderr);
+  });
+
+  // Test cases that result in buffer encodings.
+  [undefined, null, 'buffer', 'invalid'].forEach((encoding) => {
+    run({ encoding }, (stdout, stderr) => {
+      assert(stdout instanceof Buffer);
+      assert(stdout instanceof Buffer);
+      assert.strictEqual(stdout.toString(), expectedStdout);
+      assert.strictEqual(stderr.toString(), expectedStderr);
+    });
+  });
+}