lib/syncthing: Clean up / refactor LoadOrGenerateCertificate() utility function. (#8025)

LoadOrGenerateCertificate() takes two file path arguments, but then
uses the locations package to determine the actual path.  Fix that
with a minimally invasive change, by using the arguments instead.
Factor out GenerateCertificate().

The only caller of this function is cmd/syncthing, which passes the
same values, so this is technically a no-op.

* lib/tlsutil: Make storing generated certificate optional.  Avoid
  temporary cert and key files in tests, keep cert in memory.

lib/api/api_test.go

@@ -1209,15 +1209,9 @@ func TestPrefixMatch(t *testing.T) {
 }
 
 func TestShouldRegenerateCertificate(t *testing.T) {
-	dir, err := ioutil.TempDir("", "syncthing-test")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(dir)
-
 	// Self signed certificates expiring in less than a month are errored so we
 	// can regenerate in time.
-	crt, err := tlsutil.NewCertificate(filepath.Join(dir, "crt"), filepath.Join(dir, "key"), "foo.example.com", 29)
+	crt, err := tlsutil.NewCertificateInMemory("foo.example.com", 29)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1226,7 +1220,7 @@ func TestShouldRegenerateCertificate(t *testing.T) {
 	}
 
 	// Certificates with at least 31 days of life left are fine.
-	crt, err = tlsutil.NewCertificate(filepath.Join(dir, "crt"), filepath.Join(dir, "key"), "foo.example.com", 31)
+	crt, err = tlsutil.NewCertificateInMemory("foo.example.com", 31)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1236,7 +1230,7 @@ func TestShouldRegenerateCertificate(t *testing.T) {
 
 	if runtime.GOOS == "darwin" {
 		// Certificates with too long an expiry time are not allowed on macOS
-		crt, err = tlsutil.NewCertificate(filepath.Join(dir, "crt"), filepath.Join(dir, "key"), "foo.example.com", 1000)
+		crt, err = tlsutil.NewCertificateInMemory("foo.example.com", 1000)
 		if err != nil {
 			t.Fatal(err)
 		}