test: refactor test-fs-buffer

• Remove unneeded temp dir cleanup
• Add check for error in .close() callback
• Improve error reporting

On that last bullet point, the previous version of the test reported errors like this:

AssertionError: [ '.empty-repl-history-file',
  '.node_repl_history',
  'GH-1899-output.js',
  'GH-892-request.js',
  'a.js',
  'a1.js',
  'agen deepStrictEqual [ '.empty-repl-history-file',
  '.node_repl_history',
  'GH-1899-output.js',
  'GH-892-request.js',
  'a.js',
  'a1.js',
  'agen

Now, they look like this:

AssertionError: 2a is hex value for * and not catch-stdout-error.js

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test fs