Fix: Rand generator is not set in a test suite which result in accessing nil pointer during runtime if run the only test suite

[pancak3]

Reproduce: CGO_ENABLED=1 ginkgo -ldflags="-extldflags '-L/path/to/llm-d-inference-sim/lib'" -v -r -- -ginkgo.focus="Check random latencies"

click to see

Running Suite: VllmSim Suite - /workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim
======================================================================================
Random Seed: 1756093008

Will run 14 of 97 specs
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
------------------------------
Simulator Check random latencies should calculate inter token latency correctly interTokenLatency: 1000 stddev: 300
/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:770
  [PANICKED] in [It] - /usr/lib/golang/src/runtime/panic.go:262 @ 08/25/25 03:36:54.26
• [PANICKED] [0.003 seconds]
Simulator Check random latencies should calculate inter token latency correctly [It] interTokenLatency: 1000 stddev: 300
/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:770

  [PANICKED] Test Panicked
  In [It] at: /usr/lib/golang/src/runtime/panic.go:262 @ 08/25/25 03:36:54.26

  runtime error: invalid memory address or nil pointer dereference

  Full Stack Trace
    math/rand.(*Rand).Int63(...)
    	/usr/lib/golang/src/math/rand/rand.go:96
    math/rand.(*Rand).Uint32(...)
    	/usr/lib/golang/src/math/rand/rand.go:99
    math/rand.(*Rand).NormFloat64(0x0)
    	/usr/lib/golang/src/math/rand/normal.go:39 +0x24
    github.com/llm-d/llm-d-inference-sim/pkg/common.RandomNorm(0x408f400000000000, 0x4072c00000000000)
    	/workspaces/llm-d-inference-sim/pkg/common/utils.go:238 +0xdc
    github.com/llm-d/llm-d-inference-sim/pkg/llm-d-inference-sim.(*VllmSimulator).getInterTokenLatency(...)
    	/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator.go:669
    github.com/llm-d/llm-d-inference-sim/pkg/llm-d-inference-sim.init.func3.18.2(0x0?, 0x0?)
    	/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:763 +0x6c
    reflect.Value.call({0x1baf160?, 0x400051de40?, 0x13?}, {0x1e50f16, 0x4}, {0x4000848b10, 0x2, 0x2?})
    	/usr/lib/golang/src/reflect/value.go:584 +0x978
    reflect.Value.Call({0x1baf160?, 0x400051de40?, 0x2167d40?}, {0x4000848b10?, 0x0?, 0x0?})
    	/usr/lib/golang/src/reflect/value.go:368 +0x94
------------------------------
S [SKIPPED] [0.000 seconds]Running Suite: VllmSim Suite - /workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim
======================================================================================
Random Seed: 1756093008

Will run 14 of 97 specs
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
------------------------------
Simulator Check random latencies should calculate inter token latency correctly interTokenLatency: 1000 stddev: 300
/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:770
  [PANICKED] in [It] - /usr/lib/golang/src/runtime/panic.go:262 @ 08/25/25 03:36:54.26
• [PANICKED] [0.003 seconds]
Simulator Check random latencies should calculate inter token latency correctly [It] interTokenLatency: 1000 stddev: 300
/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:770

  [PANICKED] Test Panicked
  In [It] at: /usr/lib/golang/src/runtime/panic.go:262 @ 08/25/25 03:36:54.26

  runtime error: invalid memory address or nil pointer dereference

  Full Stack Trace
    math/rand.(*Rand).Int63(...)
    	/usr/lib/golang/src/math/rand/rand.go:96
    math/rand.(*Rand).Uint32(...)
    	/usr/lib/golang/src/math/rand/rand.go:99
    math/rand.(*Rand).NormFloat64(0x0)
    	/usr/lib/golang/src/math/rand/normal.go:39 +0x24
    github.com/llm-d/llm-d-inference-sim/pkg/common.RandomNorm(0x408f400000000000, 0x4072c00000000000)
    	/workspaces/llm-d-inference-sim/pkg/common/utils.go:238 +0xdc
    github.com/llm-d/llm-d-inference-sim/pkg/llm-d-inference-sim.(*VllmSimulator).getInterTokenLatency(...)
    	/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator.go:669
    github.com/llm-d/llm-d-inference-sim/pkg/llm-d-inference-sim.init.func3.18.2(0x0?, 0x0?)
    	/workspaces/llm-d-inference-sim/pkg/llm-d-inference-sim/simulator_test.go:763 +0x6c
    reflect.Value.call({0x1baf160?, 0x400051de40?, 0x13?}, {0x1e50f16, 0x4}, {0x4000848b10, 0x2, 0x2?})
    	/usr/lib/golang/src/reflect/value.go:584 +0x978
    reflect.Value.Call({0x1baf160?, 0x400051de40?, 0x2167d40?}, {0x4000848b10?, 0x0?, 0x0?})
    	/usr/lib/golang/src/reflect/value.go:368 +0x94
------------------------------
S [SKIPPED] [0.000 seconds]

Added make flat to run filtered cases: make test GINKGO_FOCUS="Check random latencies"

[irar2]

/lgtm
/approve