tailsamplingprocessor: Fix UT issue after mutator memory allocations PR

[jiekun]

Description: In #27889, we have a optimization thanks to @brancz . The optimization introduce a new field for tailSamplingSpanProcessor, which is not initialized during unit test, and cause panic.

--- FAIL: TestTraceIntegrity (0.00s)
panic: runtime error: index out of range [0] with length 0 [recovered]
	panic: runtime error: index out of range [0] with length 0

goroutine 15 [running]:
testing.tRunner.func1.2({0x112df00, 0xc00016a5a0})
	/opt/hostedtoolcache/go/1.20.10/x64/src/testing/testing.go:1526 +0x372
testing.tRunner.func1()
	/opt/hostedtoolcache/go/1.20.10/x64/src/testing/testing.go:1529 +0x650
panic({0x112df00, 0xc00016a5a0})
	/opt/hostedtoolcache/go/1.20.10/x64/src/runtime/panic.go:890 +0x263
github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor.(*tailSamplingSpanProcessor).makeDecision(0xc000326180, {0x1, 0x2, 0x3, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...)
	/home/runner/work/opentelemetry-collector-contrib/opentelemetry-collector-contrib/processor/tailsamplingprocessor/processor.go:305 +0x14b9
github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor.(*tailSamplingSpanProcessor).samplingPolicyOnTick(0xc000326180)

root cause code block:

        // We use this field here, which is not initialized in unit test, so it's still a `nil`
	mutators := tsp.mutatorsBuf
	for i, p := range tsp.policies {
                ...
                        // panic here.
			mutators[0] = tagUpsertSampled

I simply add 1 line to related test cases to have them fixed and could be merge into upstream quicker.

	tsp := &tailSamplingSpanProcessor{
                ...

                // Here
		mutatorsBuf:     make([]tag.Mutator, 1),
	}

Link to tracking Issue: Check the GitHub Action log of #27889

Testing: It's a fix for the unit test so I think no extra test case needed.

Documentation: Doc and changelog already added by @brancz in original PR. No documentation needed in this PR.

[jiekun]

Alright sorry for the lint issue. Let me run it locally and push again :/