PersonAccountId inside a test class

Question

Let's say we've got formula field on Account like this

IF (IsPersonAccount , CASESAFEID( PersonContactId ) ,"")

Is it possible to create somehow an Account record with a particular PersonContactId inside @TestSetup class? I know we are able to apply

RecordTypeID = [select Id from RecordType where (Name='Person Account') and (SobjectType='Account')].Id,

for a new created Account but there is a need to cover this code

 Account[] personAccountIds = [SELECT Personcontact_Id__c FROM  account WHERE id = :accountId];
        if(personAccountIds.size() == 0) return null;
        Id personAccountId = personAccountIds[0].Personcontact_Id__c;
        String token = getToken();

of cuz i can provide a null value but need to cover non-empty as well

Actually we need to get person contact id to retrieve data from an external service. It wasnt my decision to compose a formula field named Personcontact_Id__c to store this id. So question is - how can i test my soql with positive PersonContactId field

UPDATED In customer's org there are both Business and Person Account. Prior to me they decided to include new formula field named Personcontact_Id with formula

IF (IsPersonAccount , CASESAFEID( PersonContactId ) ,"")

Here is an APEX side method

@AuraEnabled(cacheable=true)
public static String getNext(Integer page, Integer pageSize, String accountId){
    try {
        Account[] personAccountIds = [SELECT Personcontact_Id__c FROM  account WHERE id = :accountId];
        if(personAccountIds.size() == 0) return null;
        Id personAccountId = personAccountIds[0].Personcontact_Id__c;
        String token = getToken();
        HttpResponse response = new MCCallout('MCCredentialGetData')
            .getHistory(new PayloadWrapper(token, personAccountId));
        return(String.valueOf(response.getBody()));    
    } catch (Exception e) {
        throw new AuraHandledException(e.getMessage());
    }
}

There is need to compose test class. My attempt covers the only empty (false positive test)

 @IsTest
static void getNextTestFalsePositive() {
    Assert.isNull(CXDLazyLoadingController.getNext(0, 0, '0012z00000TjF2bFFFF'));
}

So my question is how to create (or convert contact to Person Account) to be able to provide PersonContactId to test class