DUB "Error: only one `main` allowed."

[tastyminerals]

I would like to trigger tests in a simple dub project.

```
source/my_script.d
dub.json
```

Here is a dub config:
```json
{
     "targetPath": "build",
	"targetType": "executable",
	"sourcePaths": ["source"],
	"name": "my_script",
	"buildTypes": {
		"release": {
			"buildOptions": [
				"releaseMode",
				"inline",
				"optimize"
			]
		},
		"tests": {
			"buildOptions": [
				"unittests"
			]
		}
	}
}

```
The project builds but when I attempt to run `dub test`, I get

```
.dub/code/my_script-test-application-unittest-posix.osx.darwin-aarch64.arm_hardfloat-ldc_v1.26.0-8A5B544D5AC6B47B68DE875ACB4BA60E_dub_test_root.d(9,12): Error: only one `main` allowed. Previously found `main` at source/my_script.d(131,6)
```

How can one run tests with dub?