Fix BatchSpanProcessor context done tests

[MrAlias]

#1860 added support for the BatchSpanProcessor to return export failures. This was added using a select statement.

opentelemetry-go/sdk/trace/batch_span_processor.go

Lines 165 to 169 in e399d35

	select {
	case err = <-wait:
	case <-ctx.Done():
	err = ctx.Err()
	}

In Go select statements are random when multiple cases can be satisfied. This means that if the exporter is shutdown fast enough and the context has ended there is a chance that the exporter error value will be returned instead of the context error, causing test failures.

This updates the tests to have the exporter return the same context error, which a valid exporter should be doing, and resolves the bug.

[MrAlias]

Merging prior to 1 business day to avoid other test failures.