Use unicode invariant locale in __changeCase function

[vlsi]

Use case

__changeCase was introduced in #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
           |  <org.apache.jmeter.functions.ChangeCase@42db1106 values=[org.apache.jmeter.engine.util.CompoundVariable@73ae7e9c, org.apache.jmeter.engine.util.CompoundVariable@66f7d246]>
           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