NB: I consulted other similar questions, but all of them resolved clearing cache, setting APP_ENV in the xml, and/or creating the .env.testing.
Laravel version: "laravel/framework": "5.8.*",
I've the standard .env file with
APP_ENV=local
DB_CONNECTION=mysql
I've cloned the standard .env file and saved as .env.testing, then changed to 
APP_ENV=testing
DB_CONNECTION=mongodb
From console, from inside the root of the project, I'm trying to run test using
php artisan config:cache && vendor/bin/phpunit
The problem is that Laravel/PHPUnit is still using the .env file, instead of the expected .env.testing. This is the proof
TypeError: Argument 1 passed to Jenssegers\Mongodb\Query\Builder::__construct() must be an instance of Jenssegers\Mongodb\Connection, instance of Illuminate\Database\MySqlConnection given,
This is the actual phpunit.xml config file
<?xml version="1.0" encoding="UTF-8"?>
<phpunit backupGlobals="false"
         backupStaticAttributes="false"
         bootstrap="vendor/autoload.php"
         colors="true"
         convertErrorsToExceptions="true"
         convertNoticesToExceptions="true"
         convertWarningsToExceptions="true"
         processIsolation="false"
         stopOnFailure="false">
    <testsuites>
        <testsuite name="Unit">
            <directory suffix="Test.php">./tests/Unit</directory>
        </testsuite>
        <testsuite name="Feature">
            <directory suffix="Test.php">./tests/Feature</directory>
        </testsuite>
    </testsuites>
    <filter>
        <whitelist processUncoveredFilesFromWhitelist="true">
            <directory suffix=".php">./app</directory>
        </whitelist>
    </filter>
    <php>
        <server name="APP_ENV" value="testing"/>
        <server name="BCRYPT_ROUNDS" value="4"/>
        <server name="CACHE_DRIVER" value="array"/>
        <server name="MAIL_DRIVER" value="array"/>
        <server name="QUEUE_CONNECTION" value="sync"/>
        <server name="SESSION_DRIVER" value="array"/>
    </php>
</phpunit>
So, I kindly ask a suggestion for debugging and fixing
I found this issue: https://github.com/sebastianbergmann/phpunit/issues/2353
So I tried to change this portion of .xml file to force my configs.
It works. note all rows are now with appended force="true"
    <server name="APP_ENV" value="testing"  force="true"/>
    <server name="BCRYPT_ROUNDS" value="4" force="true"/>
    <server name="CACHE_DRIVER" value="array" force="true"/>
    <server name="MAIL_DRIVER" value="array" force="true"/>
    <server name="QUEUE_CONNECTION" value="sync" force="true"/>
    <server name="DB_CONNECTION" value="mongodb_testing" force="true"/>
    <server name="SESSION_DRIVER" value="array" force="true"/>
                        I faced this issue and I tried to update phpunit.xml to use
<env /> instead of <server />
and it works (Laravel 5.7)
<php>
    <env name="APP_ENV" value="testing"/>
    <env name="APP_DEBUG" value="false"/>
    <env name="CACHE_DRIVER" value="array"/>
    <env name="DB_CONNECTION" value="sqlite"/>
    <env name="DB_DATABASE" value=":memory:"/>
    <env name="MAIL_MAILER" value="array"/>
    <env name="QUEUE_CONNECTION" value="sync"/>
    <env name="SESSION_DRIVER" value="array"/>
</php>
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