test: convert assert.equal to assert.strictEqual

converts an instance of assert.equal in a file already mostly
updated to use assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/9925
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-slow.js

@@ -30,7 +30,7 @@ assert.strictEqual(SlowBuffer(0).length, 0);
 try {
   assert.strictEqual(SlowBuffer(buffer.kMaxLength).length, buffer.kMaxLength);
 } catch (e) {
-  assert.equal(e.message, 'Invalid array buffer length');
+  assert.strictEqual(e.message, 'Invalid array buffer length');
 }
 
 // should work with number-coercible values