When explicitly specifying the `execute` subcommand as advised by:
> WARNING: Delegated to the 'execute' command.
>          This behaviour has been deprecated and will be removed in a future release.
>          Please use the 'execute' command directly.

... it happens that `ConsoleLauncher.run` returns a `CommandResult`
whose `getValue` method always returns `Optional.empty()` instead of the
expected (non-empty) `Optional<TestExecutionSummary>`.

It turns out that, when `picocli` executes a _subcommand_, it doesn't
propagate the return value to the parent `CommandLine`, whose
`getExecutionResult` method then returns `null`.

There was a question about it last year
(remkop/picocli#1656) answered by the [user
manual](https://picocli.info/#_executing_commands_with_subcommands):
> The `ParseResult` can be used to get the return value from a Callable
> or Method subcommand:
> ```java
> // getting return value from Callable or Method command
> Object topResult = cmd.getExecutionResult();
>
> // getting return value from Callable or Method subcommand
> ParseResult parseResult = cmd.getParseResult();
> if (parseResult.subcommand() != null) {
>    CommandLine sub = parseResult.subcommand().commandSpec().commandLine();
>    Object subResult = sub.getExecutionResult();
> }
> ```

The present change therefore consists in implementing it.

Note: prior to the change, presently adapted tests (now parameterized so
as to cover both forms) were all failing on:
```java
java.lang.IllegalStateException: TestExecutionSummary not assigned. Exit code is: 0
	at app//org.junit.platform.console.ConsoleLauncherWrapperResult.checkTestExecutionSummaryState(ConsoleLauncherWrapperResult.java:42)
	at app//org.junit.platform.console.ConsoleLauncherWrapperResult.getTestsFoundCount(ConsoleLauncherWrapperResult.java:102)
```