streams/: Fix issues found by eslint

[ricea]

There is no requirement for tests to pass eslint, but I find it a useful tool
for debugging so I cleaned up a few non-controversial issues in the streams/
test directory.

Fixed:

• Unused variables
• Missing '()' invoking a constructor
• 'cancelPromise' is never reassigned. Use 'const' instead
• Missing semicolon
• '+' should be placed at the end of the line (subjective, but probably nobody
  cares enough to object)
• This line has 2 statements. Maximum allowed is 1
• Use the rest parameters instead of 'arguments'

Not changed:

• Variable 'ignoredReadPromise' is unused on purpose
• Lines > 120 (no universal agreement on this)
• Arrow function should not return assignment (no universal agreement)
• A space is required after '{' / before '}' (no universal agreement)

[wpt-pr-bot]

Notifying @calvaris, @domenic, @tyoshino, @wanderview, @youennf, and @yutakahirano. (Learn how reviewing works.)