$ docker run -it --rm -v "$PWD:$PWD" -w "$PWD" swift:5.1.3 swift test --enable-test-discovery
/tmp/repro/.build/x86_64-unknown-linux/debug/testlist.derived/reproTests.swift:11:16: error: invalid redeclaration of '__allTests__reproTests'
    static let __allTests__reproTests = [
               ^
/tmp/repro/.build/x86_64-unknown-linux/debug/testlist.derived/reproTests.swift:5:16: note: '__allTests__reproTests' previously declared here
    static let __allTests__reproTests = [
               ^
[10/12] Compiling reproPackageTests main.swift

$ tree Tests/
Tests/
├── LinuxMain.swift
└── reproTests
    ├── otherFile.swift
    └── reproTests.swift

$ cat Tests/reproTests/reproTests.swift 
import XCTest
@testable import repro

final class reproTests: XCTestCase {
    func testExample() {
        // This is an example of a functional test case.
        // Use XCTAssert and related functions to verify your tests produce the correct
        // results.
        XCTAssertEqual(repro().text, "Hello, World!")
    }
}

$ cat Tests/reproTests/otherFile.swift 
import Foundation

extension reproTests {
    func testSomthingInOtherFile() {
        
    }
}