Feature: GstTest in Background

Launch a gst pipeline in background for test cases with multiple
gst pipelines collaborating with each other.

Example usage:

gstTestBackground "background pipeline description" TCNAME 0 0 5
// launching the pipeline with 5s timeout for launching-timeout
// Note that the timeout only waits for the pipeline initialization.
// The pipeline shouldn't have indefinite prerolling wait.
// If this pipeline waits for inputs after the launch, make it
// async=false (at sink) so that it may skip prerolling

gstTest "foreground pipeline description, collaborating with above" ...

wait $pid
// the global variable "pid" is the PID of the background pipeline.

// alternatively
kill $pid // the background pipeline is no more needed.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>