software/{erp5,jstestnode}: define default for testrunner oneOf schemas

https://rjsf-team.github.io/react-jsonschema-form/ can handle this
correctly, but needs a default along with the const.

software/erp5/instance-erp5-input-schema.json

@@ -565,7 +565,8 @@
                 "target": {
                   "description": "Target system",
                   "type": "string",
-                  "const": "selenium-server"
+                  "const": "selenium-server",
+                  "default": "selenium-server"
                 },
                 "server-url": {
                   "description": "URL of the selenium server",

software/jstestnode/instance-jstestnode-input-schema.json

@@ -64,7 +64,8 @@
             "target": {
               "description": "Target system",
               "type": "string",
-              "const": "selenium-server"
+              "const": "selenium-server",
+              "default": "selenium-server"
             },
             "server-url": {
               "description": "URL of the selenium server",
@@ -128,7 +129,8 @@
             "target": {
               "description": "Target system",
               "const": "node",
-              "type": "string"
+              "type": "string",
+              "default": "node"
             }
           }
         }