phraser, mavend: read the fallbacks from the file (V-501)

The accessors are functions now, so the call sites that compared against one
literal compare against the entry instead: IsUnknownFallback and
IsSourcesFallback in the daemon tests, the entry key in the phraser tests. A
reworded variant no longer breaks a Go test.

The eval scores every variant on the persona checks the nudges already pass.

internal/phraser/fallbacks_test.go

@@ -0,0 +1,57 @@
+package phraser
+
+import (
+	"math/rand"
+	"strings"
+	"testing"
+)
+
+// The embedded file must load, or the daemon speaks from hardFloor and nobody
+// finds out until he hears the wrong words.
+func TestFallbacksLoad(t *testing.T) {
+	fb, err := LoadFallbacks(rand.NewSource(1))
+	if err != nil {
+		t.Fatalf("LoadFallbacks: %v", err)
+	}
+	if got := fb.FromSources("два литра"); !strings.Contains(got, "два литра") {
+		t.Errorf("FromSources = %q, want the sources in it", got)
+	}
+	if fb.WorldGap() != hardFloor[fbWorldGap] {
+		t.Errorf("WorldGap = %q, want the fixed wording %q", fb.WorldGap(), hardFloor[fbWorldGap])
+	}
+}
+
+// A broken or missing file must not take her last words away: every accessor
+// answers from the literal it replaced.
+func TestNilFallbacksAnswerFromTheHardFloor(t *testing.T) {
+	var fb *Fallbacks
+	if got := fb.Chat(); got != hardFloor[fbChat] {
+		t.Errorf("Chat = %q, want %q", got, hardFloor[fbChat])
+	}
+	if got := fb.Unknown(); got != hardFloor[fbQueryUnknown] {
+		t.Errorf("Unknown = %q, want %q", got, hardFloor[fbQueryUnknown])
+	}
+	if got := fb.FromSources("два литра"); got != "вот что я нашла: два литра" {
+		t.Errorf("FromSources = %q", got)
+	}
+	if got := fb.WorldGap(); got != hardFloor[fbWorldGap] {
+		t.Errorf("WorldGap = %q", got)
+	}
+}
+
+// Hearing the identical words every time a request fails is how a failure stops
+// registering as one.
+func TestFallbacksDoNotRepeat(t *testing.T) {
+	fb, err := LoadFallbacks(rand.NewSource(7))
+	if err != nil {
+		t.Fatalf("LoadFallbacks: %v", err)
+	}
+	prev := fb.Chat()
+	for i := 0; i < 20; i++ {
+		got := fb.Chat()
+		if got == prev {
+			t.Fatalf("chat repeated %q on turn %d", got, i)
+		}
+		prev = got
+	}
+}