fix(import): a deleted DXCC entity is not re-stamped from cty.dat

Reported with a Logger32 export: the record said <DXCC:3>151 and OpsLog
stored 54.

cty.dat answers one question — where is this callsign TODAY — and that is
the wrong question for a log. R1MVI was Malyj Vysotskij Island, entity
151, until the ARRL deleted it in 2012; the same call resolves to European
Russia today. Forcing the lookup onto a 2004 contact therefore destroys a
credit that can NEVER be worked again, because the place no longer counts.

So the ADIF's number wins whenever it names a deleted entity, and nothing
is corrected on those records at all — the country name and the zones of a
deleted entity are equally beyond what a present-day prefix table knows.
The list is the ADIF specification's own enumeration, sixty-odd entities
that only change when the ARRL deletes another one.

The guard sits in the forced path, so it also covers the grid's 'Update
from cty.dat' on a selection — the other way an operator could have lost
the same credits, one right-click at a time.

internal/dxcc/deleted_test.go

@@ -0,0 +1,46 @@
+package dxcc
+
+import "testing"
+
+// The contact that reported this: R1MVI, worked in 2004 on Malyj Vysotskij
+// Island, entity 151, deleted in February 2012. cty.dat resolves that callsign
+// to European Russia (54) today, so anything that re-stamps an old QSO from a
+// callsign has to know to stop here.
+func TestMalyjVysotskijIsKnownDeleted(t *testing.T) {
+	if !IsDeleted(151) {
+		t.Fatal("151 (Malyj Vysotskij I.) must be known as deleted — the whole guard hangs on it")
+	}
+	if got := DeletedName(151); got == "" {
+		t.Error("a deleted entity should be able to say what it was called")
+	}
+}
+
+// The guard must not fire on entities that are alive, or every log would freeze
+// at whatever DXCC number it was imported with — including the wrong ones.
+func TestLiveEntitiesAreNotTreatedAsDeleted(t *testing.T) {
+	live := map[int]string{
+		54:  "European Russia — what R1MVI resolves to today",
+		227: "France",
+		291: "United States",
+		339: "Japan",
+		1:   "Canada",
+	}
+	for n, what := range live {
+		if IsDeleted(n) {
+			t.Errorf("%d (%s) must not be listed as deleted", n, what)
+		}
+	}
+	if got := DeletedName(54); got != "" {
+		t.Errorf("DeletedName(54) = %q, want empty", got)
+	}
+}
+
+// A few more from the ADIF enumeration, so a careless edit to the table is
+// caught rather than discovered by an operator whose credits moved.
+func TestSomeOtherDeletedEntities(t *testing.T) {
+	for _, n := range []int{218 /* Czechoslovakia */, 229 /* German DR */, 255 /* St. Maarten, Saba, St. Eustatius */, 28 /* Canal Zone */} {
+		if !IsDeleted(n) {
+			t.Errorf("%d should be a deleted entity", n)
+		}
+	}
+}