add tests for sender, receiver open errors

[ortex]

#67

[coveralls]

Coverage: 87.335% (+1.5%) from 85.822% when pulling b7608bc on ortex:sender-receiver-open-test-errors into e4be31a on roc-streaming:main.

[gavv]

We could nicely merge new tests into TestXxx_Open by adding context creation func to the table:

		{
			name:    "closed context",
			...
			contextFunc: func() *Context {
				ctx, err := OpenContext()
				ctx.Close()
				return ctx
			},
		},

WDYT?

[gavv]

LGTM!