test: remove literal error messages

Assertions will now print the values that caused the assertions
to fail.

PR-URL: nodejs#15928
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-zlib-flush-drain.js

@@ -38,13 +38,11 @@ deflater.on('drain', function() {
 
 process.once('exit', function() {
   assert.strictEqual(
-    beforeFlush, true,
-    'before calling flush, writable stream should need to drain');
+    beforeFlush, true);
   assert.strictEqual(
-    afterFlush, false,
-    'after calling flush, writable stream should not need to drain');
+    afterFlush, false);
   assert.strictEqual(
-    drainCount, 1, 'the deflater should have emitted a single drain event');
+    drainCount, 1);
   assert.strictEqual(
-    flushCount, 2, 'flush should be called twice');
+    flushCount, 2);
 });