Split XML gen action should have different mnemonic

### Description of the feature request:

It should not use the same TestRunner mnemonic, which is very confusing for newer users.

It would be best to have a separate execution group for this as well so that when running on RBE, users can use a different set of exec properties for this action (i.e. when running bazel coverage, the default exec properties of TestRunner action may not include gcov, while the split gen xml action does require gcov installed to run successfully.)

### Which category does this issue belong to?

_No response_

### What underlying problem are you trying to solve with this feature?

Making it easier for Build Event UI users to tell split xml gen action apart from TestRunner actions.

Also make coverage a bit easier to configure.

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

Should still be relevant as of Bazel 9

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

Probably need to start here

https://github.com/bazelbuild/bazel/blob/156c627cf551bb3e15f908fc88f0f5595a796d8f/src/main/java/com/google/devtools/build/lib/exec/StandaloneTestStrategy.java#L473-L485

and make sure we use a new action instead of the parent TestRunner action to create the spawn?

Start in StandaloneTestStrategy.java at the linked lines and trace how the split gen XML spawn is created from the parent TestRunner action. Determine where its mnemonic and execution group are assigned; done means the split action is distinguishable in the Build Event UI and can receive separate execution properties, including gcov for coverage.

Written by the indexing model from the issue text.