fix tests in iOS Simulator (#1171)
fix tests in iOS Simulator (#1171)

Motivation:

The iOS Simulators have extremely long temp directory names that contain
the user name (which varies in length). UNIX Domain Socket paths however
are limited to 103 bytes.

Modifications:

Use /tmp as the temp directory for Simulator environments.

Result:

Tests run in iOS Simulators.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio