MyProject
|-Sources
| └MyProject
|  |-IPcalc.swift
|  └ main.swift
|
|-Tests
| |-MyProjectTests
| | └IPcalcTests.swift
| └ LinuxMain.swift
|
└ Package.swift

import XCTest
@testable import IPcalc
...

import XCTest
@testable import IPcalcTests

XCTMain([
    testCase(IPcalcTests.allTests),
])

import PackageDescription

let package = Package(
    name: "MyProject",
    products: [],
    dependencies: [],
    targets: [
        .target(
            name: "MyProject",
            dependencies: []),
        .testTarget(
            name: "MyProjectTests",
            dependencies: ["MyProject"]),
    ]
)

$ swift test
Compile Swift Module 'MyProjectTests' (2 sources)
/home/user/MyProject/Tests/MyProjectTests/IPcalcTests.swift:2:18: error: no such module 'IPcalc'
@testable import IPcalc