net/netcheck: ignore PreferredDERP changes that are small

If the absolute value of the difference between the current
PreferredDERP's latency and the best latency is <= 10ms, don't change
it and instead prefer the previous value.

This is in addition to the existing hysteresis that tries to remain
on the previous DERP region if the relative improvement is small, but
handles nodes that have low latency to >1 DERP region better.

Updates #8603

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I1e34c94178f8c9a68a69921c5bc0227337514c70

net/netcheck/netcheck_test.go

@@ -327,6 +327,15 @@ func TestAddReportHistoryAndSetPreferredDERP(t *testing.T) {
 			wantPrevLen: 2,
 			wantDERP:    1, // 2 didn't get fast enough
 		},
+		{
+			name: "preferred_derp_hysteresis_no_switch_absolute",
+			steps: []step{
+				{0 * time.Second, report("d1", 4*time.Millisecond, "d2", 5*time.Millisecond)},
+				{1 * time.Second, report("d1", 4*time.Millisecond, "d2", 1*time.Millisecond)},
+			},
+			wantPrevLen: 2,
+			wantDERP:    1, // 2 is 50%+ faster, but the absolute diff is <10ms
+		},
 		{
 			name: "preferred_derp_hysteresis_do_switch",
 			steps: []step{