test: move NUMA platform scan out of testing global

[tgross]

The testing.go test helpers file for the driver manager initializes the NUMA scan as a package-global variable. This causes it to be pulled in even in production builds, so even running commands like nomad version will cause the NUMA scan to happen. Move the scan into the test helper setup.

I discovered this while debugging #23284 but this suggests there may be other cases where our testing.go test helper files are dragging wasteful startup behaviors into production builds.

---

Before:

$ objdump -t $(which nomad) | grep topology
00000000011096e0 l     F .text  00000000000003d0              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyGeneric
0000000001109e60 l     F .text  00000000000000a5              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacy
0000000001109f20 l     F .text  0000000000000314              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyLinux
000000000110a240 l     F .text  0000000000000066              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyLinux.func1
0000000004ba9560 l     O .bss   0000000000000008              github.com/hashicorp/nomad/client/pluginmanager/drivermanager.topology

After:

$ objdump -t $(which nomad) | grep topology
00000000011096e0 l     F .text  00000000000003d0              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyGeneric
0000000001109e60 l     F .text  00000000000000a5              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacy
0000000001109f20 l     F .text  0000000000000314              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyLinux
000000000110a240 l     F .text  0000000000000066              github.com/hashicorp/nomad/nomad/structs.topologyFromLegacyLinux.func1

[pkazmierczak]

LGTM!

[github-actions]

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