expvar can only be defined once, so running tests with a repeat counter
will fail if the variables are defined inside of the test function.
Observed failure:
```
--- FAIL: TestHandler (0.00s)
panic: Reuse of exported var name: gauge_promvarz_test_expvar
[recovered]
panic: Reuse of exported var name: gauge_promvarz_test_expvar
goroutine 9 [running]:
testing.tRunner.func1.2({0x100f267e0, 0x1400026e770})
/usr/local/go/src/testing/testing.go:1526 +0x1c8
testing.tRunner.func1()
/usr/local/go/src/testing/testing.go:1529 +0x364
panic({0x100f267e0, 0x1400026e770})
/usr/local/go/src/runtime/panic.go:884 +0x1f4
log.Panicln({0x140000b8e20?, 0x1a?, 0x1400026e750?})
/usr/local/go/src/log/log.go:398 +0x60
expvar.Publish({0x100e2b21d, 0x1a}, {0x100fd7a08?, 0x140000232c0})
/usr/local/go/src/expvar/expvar.go:284 +0xc0
expvar.NewInt(...)
/usr/local/go/src/expvar/expvar.go:304
tailscale.com/tsweb/promvarz.TestHandler(0x14000082b60)
/Users/charlotte/ts-src/tailscale/tsweb/promvarz/promvarz_test.go:18 +0x5c
testing.tRunner(0x14000082b60, 0x100fd5858)
/usr/local/go/src/testing/testing.go:1576 +0x104
created by testing.(*T).Run
/usr/local/go/src/testing/testing.go:1629 +0x370
FAIL tailscale.com/tsweb/promvarz 0.149s
```
Fixes#8065
Signed-off-by: James Tucker <james@tailscale.com>
5c38f0979e