lib: consistently use f-tests instead of table-driven tests

This makes easier to read and debug these tests. This also reduces test lines count by 15% from 3K to 2.5K
See https://itnext.io/f-tests-as-a-replacement-for-table-driven-tests-in-go-8814a8b19e9e

While at it, consistently use t.Fatal* instead of t.Error*, since t.Error* usually leads
to more complicated and fragile tests, while it doesn't bring any practical benefits over t.Fatal*.

lib/httpserver/httpserver_test.go

@@ -135,9 +135,13 @@ func TestAuthKeyMetrics(t *testing.T) {
 }
 
 func TestHandlerWrapper(t *testing.T) {
-	*headerHSTS = "foo"
-	*headerFrameOptions = "bar"
-	*headerCSP = "baz"
+	const hstsHeader = "foo"
+	const frameOptionsHeader = "bar"
+	const cspHeader = "baz"
+
+	*headerHSTS = hstsHeader
+	*headerFrameOptions = frameOptionsHeader
+	*headerCSP = cspHeader
 	defer func() {
 		*headerHSTS = ""
 		*headerFrameOptions = ""
@@ -152,13 +156,14 @@ func TestHandlerWrapper(t *testing.T) {
 		return true
 	})
 
-	if w.Header().Get("Strict-Transport-Security") != "foo" {
-		t.Errorf("HSTS header not set")
+	h := w.Header()
+	if got := h.Get("Strict-Transport-Security"); got != hstsHeader {
+		t.Fatalf("unexpected HSTS header; got %q; want %q", got, hstsHeader)
 	}
-	if w.Header().Get("X-Frame-Options") != "bar" {
-		t.Errorf("X-Frame-Options header not set")
+	if got := h.Get("X-Frame-Options"); got != frameOptionsHeader {
+		t.Fatalf("unexpected X-Frame-Options header; got %q; want %q", got, frameOptionsHeader)
 	}
-	if w.Header().Get("Content-Security-Policy") != "baz" {
-		t.Errorf("CSP header not set")
+	if got := h.Get("Content-Security-Policy"); got != cspHeader {
+		t.Fatalf("unexpected CSP header; got %q; want %q", got, cspHeader)
 	}
 }