tlsutil: fix default server name for health checks

Don't use the agent node name or agent server name when EnableAgentTLSForChecks=false.

.changelog/10490.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+checks: fixes the default ServerName used with TLS health checks.
+```

tlsutil/config.go

@@ -720,10 +720,6 @@ func (c *Configurator) IncomingHTTPSConfig() *tls.Config {
 func (c *Configurator) OutgoingTLSConfigForCheck(skipVerify bool, serverName string) *tls.Config {
 	c.log("OutgoingTLSConfigForCheck")
 
-	if serverName == "" {
-		serverName = c.serverNameOrNodeName()
-	}
-
 	if !c.enableAgentTLSForChecks() {
 		return &tls.Config{
 			InsecureSkipVerify: skipVerify,
@@ -731,6 +727,9 @@ func (c *Configurator) OutgoingTLSConfigForCheck(skipVerify bool, serverName str
 		}
 	}
 
+	if serverName == "" {
+		serverName = c.serverNameOrNodeName()
+	}
 	config := c.commonTLSConfig(false)
 	config.InsecureSkipVerify = skipVerify
 	config.ServerName = serverName

tlsutil/config_test.go

@@ -948,6 +948,34 @@ func TestConfigurator_OutgoingTLSConfigForCheck(t *testing.T) {
 			skipVerify: true,
 			expected:   &tls.Config{InsecureSkipVerify: true},
 		},
+		{
+			name: "default tls, skip verify, default server name",
+			conf: func() (*Configurator, error) {
+				return NewConfigurator(Config{
+					TLSMinVersion:           "tls12",
+					EnableAgentTLSForChecks: false,
+					ServerName:              "servername",
+				}, nil)
+			},
+			skipVerify: true,
+			expected:   &tls.Config{InsecureSkipVerify: true},
+		},
+		{
+			name: "default tls, skip verify, check server name",
+			conf: func() (*Configurator, error) {
+				return NewConfigurator(Config{
+					TLSMinVersion:           "tls12",
+					EnableAgentTLSForChecks: false,
+					ServerName:              "servername",
+				}, nil)
+			},
+			skipVerify: true,
+			serverName: "check-server-name",
+			expected: &tls.Config{
+				InsecureSkipVerify: true,
+				ServerName:         "check-server-name",
+			},
+		},
 		{
 			name: "agent tls, skip verify, default server name",
 			conf: func() (*Configurator, error) {