Test `InstallTraceBridge` in the `opencensus` bridge

[MrAlias]

opentelemetry-go/bridge/opencensus/trace.go

Lines 15 to 20 in dbcd24f

	// InstallTraceBridge installs the OpenCensus trace bridge, which overwrites
	// the global OpenCensus tracer implementation. Once the bridge is installed,
	// spans recorded using OpenCensus are redirected to the OpenTelemetry SDK.
	func InstallTraceBridge(opts ...TraceOption) {
	octrace.DefaultTracer = newTraceBridge(opts)
	}

The test should verify that the octrace.DefaultTracer is correctly set.

[zaouldyeck]

Working on this one.