TYPO3 Core

I have the same issue. I installed a Typo3 instance using following composer.json (just the relevant parts):

{
    "name": "my/typo3-install",
    "repositories": [
        {
            "type": "composer",
            "url": "https://composer.typo3.org/" 
        }
    ],
    "require": {
        "typo3/cms": "~7.6" 
    },
    "require-dev": {
        "phpunit/phpunit": "^5.5" 
    },
    "autoload-dev": {
        "psr-4": {
            "TYPO3\\CMS\\Core\\Tests\\": "typo3/sysext/core/Tests/",
            "TYPO3\\CMS\\Extbase\\Tests\\": "typo3/sysext/extbase/Tests/" 
        },
        "classmap": ["typo3/sysext/extbase/Tests/Unit/Object/Container/Fixtures/"]
    }
}

I wanted to add some unit tests to my extension but as said by Sebastian the "Tests" folder of extbase (typo3/sysext/extbase/Tests/) simply does not exist.
Is it intentionally excluded from the Typo3 composer package?

With removing the line

"classmap": ["typo3/sysext/extbase/Tests/Unit/Object/Container/Fixtures/"]

It works for me.

I still experience the error with 7.6.26:

    composer create-project typo3/cms typo3-7 ^7

but as Anja described in #81118 the parameter --prefer-source avoids the situation where the error is raised.