Remove a debug! statement before I/O is ready

The logging macros all use libuv-based I/O, and there was one stray debug
statement in task::spawn which was executing before the I/O context was ready.
Remove it and add a test to make sure that we can continue to debug this sort of
code.

Closes #10405
auto merge of #10406 : alexcrichton/rust/issue-10405, r=huonw

The logging macros all use libuv-based I/O, and there was one stray debug
statement in task::spawn which was executing before the I/O context was ready.
Remove it and add a test to make sure that we can continue to debug this sort of
code.

Closes #10405