When two test suites have the same name, one of them will be overriden

[Aboisier]

This problem stems from the loadTests method. We need to throw an error if there is a name conflict instead of blindly overrinding the previous test suite.

public async loadTests(root: string, patterns: string[], tsconfig: tsnode.Options): Promise<TestSuiteInstance> {
    this.registerTranspiler(tsconfig);

    const testSuites = new RootTestSuite();
    const files = await this.getTestFiles(root, patterns);
    for (const file of files) {
        const testInstances = await this.getTestInstancesFromFile(file);
        testInstances.forEach(x => testSuites.set(x.name, x));
    }
    
    return testSuites;
}