When you add a custom environment configuration, you need to add it also to the test element in the angular.json file, otherwise tests just run with the default environment configuration.

For instance, lets say you added an extra environment configuration named "customenv" to the already existing build.configurations element:

Copy"build": {
    ...

    "configurations": {
        "production": {
            "budgets": [
                {
                    "type": "initial",
                    "maximumWarning": "500kb",
                    "maximumError": "1mb"
                },
                {
                    "type": "anyComponentStyle",
                    "maximumWarning": "2kb",
                    "maximumError": "4kb"
                }
            ],
            "fileReplacements": [
                {
                    "replace": "src/environments/environment.ts",
                    "with": "src/environments/environment.prod.ts"
                }
            ],
            "outputHashing": "all"
        },
        "development": {
            "buildOptimizer": false,
            "optimization": false,
            "vendorChunk": true,
            "extractLicenses": false,
            "sourceMap": true,
            "namedChunks": true
        },
        "customenv": {
            "fileReplacements": [
                {
                    "replace": "src/environments/environment.ts",
                    "with": "src/environments/environment.customenv.ts"
                }
            ]
        }
    },
    "defaultConfiguration": "production"
},

At this point you can run

Copyng build --configuration=customenv

And then you might have also modified the already existing serve.configurations element like this:

Copy"serve": {
    ...
    "configurations": {
        "production": {
            "browserTarget": "myproject:build:production"
        },
        "development": {
            "browserTarget": "myproject:build:development"
        },
        "testws": {
            "browserTarget": "myproject:build:customenv"
        }
    },
    "defaultConfiguration": "development"
},

So now you can run:

Copyng serve --configuration=customenv

But if you try to run

Copyng test --configuration=customenv

You get the error:

An unhandled exception occurred: Configuration 'customenv' is not set in the workspace.

To be able to do that, you need to create a test.configurations element and add only the new custom environment:

Copy"test": {
    ...                  
    "configurations": {                        
        "customenv": {
            "fileReplacements": [
                {
                    "replace": "src/environments/environment.ts",
                    "with": "src/environments/environment.customenv.ts"
                }
            ]
        }
    },

This can be achieved these days via the include option. https://angular.io/cli/test#options

It's a glob match, so as an example:

ng test --include='**/someFolder/*.spec.ts'

I can't find it in the 8.1.0 release notes, but @Swoox mentions below this is a feature after cli version 8.1.0. Thanks for figuring that out.