all: fix six tests that failed with -count=2

Avery found a bunch of tests that fail with -count=2.

Updates tailscale/corp#40176 (tracks making our CI detect them)

Change-Id: Ie3e4398070dd92e4fe0146badddf1254749cca20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Avery Pennarun <apenwarr@tailscale.com>

tsweb/varz/varz_test.go

@@ -205,7 +205,7 @@ func TestVarzHandler(t *testing.T) {
 			"string_map",
 			func() *expvar.Map {
 				m := new(expvar.Map)
-				m.Set("a", expvar.NewString("foo"))
+				m.Set("a", new(expvar.String))
 				return m
 			}(),
 			"# skipping \"string_map\" expvar map key \"a\" with unknown value type *expvar.String\n",