Add instance variable to define a name of test generated's class

[Lin777]

Fixes #46
Now you can specify by this way:

STestCaseFactoryPharo new
    typeInfo: typeInfo;
    fitness: SStatementCoverage new;
    targetClassName:#SFoo;
    targetPackageRegex:'SmallSuiteGenerator-Scenario';
    outputPackageName:'SmallSuiteGenerator-Tests-Generated';
     numberOfGenerations: 20;
    numberOfStatements: 10;
    targetTestClassName: 'MyTEST';
    createTestCases;
    yourself.