Use unicode invariant locale in __changeCase function

### Use case

`__changeCase` was introduced in https://github.com/apache/jmeter/issues/4587, and it uses locale-dependent conversion. It makes it prone to errors.

Kotlin recently switched to invariant locale when the user provides no locale: https://youtrack.jetbrains.com/issue/KT-43023
In other words, `"i".uppercase()` yields `I` even in tr_TR locale.

Currently `org.apache.jmeter.functions.ChangeCaseSpec` fails in `tr_TR` locale as follows:

```
FAILURE 0,4sec, org.apache.jmeter.functions.ChangeCaseSpec > convert 'simple' using mode upper to 'SIMPLE'
Condition not satisfied:

output == changeCase.execute(result, null)
| | | | |
SIMPLE | | S?MPLE org.apache.jmeter.samplers.SampleResult@7cbebcbf
|
false
1 difference (83% similarity)
S(I)MPLE
S(?)MPLE
```

### Possible solution

Add an extra parameter to `__changeCase` so the user can specify the locale. A special value of `default` could designate `Locale.getDefault()`.

### Possible workarounds

Skip `ChangeCaseSpec` in `tr_TR` locale.

### JMeter Version

5.5

### Java Version

_No response_

### OS Version

_No response_

Start with org.apache.jmeter.functions.ChangeCaseSpec and the __changeCase entry point. Reproduce the failure under the tr_TR locale, then determine how locale selection should work, including the proposed default value. Done means the case-conversion behavior is covered by tests and no longer fails for the reported locale.

Written by the indexing model from the issue text.