test: improve assert messages in stream test

In test-stream-pipe-await-train-manual-resume, include unexpected value
in error messages.

PR-URL: #16884
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-stream-pipe-await-drain-manual-resume.js

@@ -30,11 +30,13 @@ readable.once('pause', common.mustCall(() => {
   assert.strictEqual(
     readable._readableState.awaitDrain,
     1,
-    'awaitDrain doesn\'t increase'
+    'Expected awaitDrain to equal 1 but instead got ' +
+    `${readable._readableState.awaitDrain}`
   );
   // First pause, resume manually. The next write() to writable will still
   // return false, because chunks are still being buffered, so it will increase
   // the awaitDrain counter again.
+
   process.nextTick(common.mustCall(() => {
     readable.resume();
   }));
@@ -43,7 +45,8 @@ readable.once('pause', common.mustCall(() => {
     assert.strictEqual(
       readable._readableState.awaitDrain,
       1,
-      '.resume() does not reset counter'
+      '.resume() should not reset the counter but instead got ' +
+      `${readable._readableState.awaitDrain}`
     );
     // Second pause, handle all chunks from now on. Once all callbacks that
     // are currently queued up are handled, the awaitDrain drain counter should
@@ -64,7 +67,8 @@ writable.on('finish', common.mustCall(() => {
   assert.strictEqual(
     readable._readableState.awaitDrain,
     0,
-    'awaitDrain not 0 after all chunks are written'
+    'awaitDrain should equal 0 after all chunks are written but instead got' +
+    `${readable._readableState.awaitDrain}`
   );
   // Everything okay, all chunks were written.
 }));