Added Timeout() for Describe and Top Level

[Neirpyc]

What was the previous behavior

Previously, calling G.Timeout() inside a Describe or in the top level test function resulted in the following error:
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
This was because of this piece of code:

func (g *G) Timeout(time time.Duration) {
	g.timeout = time
	g.timer.Reset(time) // <- g.timer not defined outside of an It block
}

Which behavior is this PR bringing

• When inside an It block, nothing changes
• When inside a Describe block, sets the timeout to the given value, except when overridden inside an It block (only affect the It inside which is the g.Timeout())
• When outside of any Describe or It it is applied to every It block unless overriden in the It or in the parent Describe.

g.Timeout() can be called multiple times in which case the given value applies until a new g.Timeout() is encountered in the same or an higher scope.

What would have been another valid behavior ?

It could be coherent that calling g.Timeout inside a Describe sets a maximum execution time for the whole Describe, and not for every It inside it. The same way, setting the timeout in the top level function could set a global time for all of the Describes instead of a time per It.

Which improvements could be made ?

The test suite for the timeouts is pretty bad, as it calls time.Sleep() many times, which makes the testing slow, and it could be interesting to think of a way to mock the Sleep calls.