In an attempt to implement a solution for this problem, I have Swift source file zShearFactor.swift that defines a function
zWarpShearFactor that returns a struct of type SVDFactors:

import Foundation
import simd

struct SVDFactors {
    let U : simd_float3x3
    let S : simd_float3x3
    let V : simd_float3x3
}

func zWarpShearFactor(a : Float, b : Float, c : Float) -> SVDFactors {
   ...
}

Trying to be the diligent programmer I ought to be I created a XCode unit test

import XCTest
import simd
@testable import zshear_factor

class zshear_factor_test: XCTestCase {
    static func verifySVD(a : Float, b : Float, c : Float, svd : SVDFactors) -> Bool {
        let A = simd_float3x3(rows: [
            simd_float3(1, 0, a),
            simd_float3(0, 1, b),
            simd_float3(0, 0, c)
        ])
        let A_ = svd.U * svd.S * simd_transpose(svd.V)
        return simd_almost_equal_elements(A, A_, 0.0001)
    }

    func testzWarpShearFactor() {
        let testValues : [Float] = [0, -1, +1, 10, -10, 1.23, -1.23]
        for a in testValues {
            for b in testValues {
                for c in testValues {
                    let SVD = zWarpShearFactor(a: a, b: b, c: c)
                    XCTAssertTrue(Self.verifySVD(a: a, b: b, c: c,
                                                 svd: SVD),
                                  "zWarpShearFactor((a),(b),(c) FAIL")
                }
            }
        }
    }
}

Even though I imported the via @testable import zshear_factor there is obviously
more I need to do since I get a linker error when I click on the diamond to
run the Xcode project:

• The accepted answer to a similar question does not work in this
  case.
• I placed this small example project on github.

The initial question is what do I put in the Build Settings so that the proper
code is linked with the unit test? The bigger question is how is this done
on very large projects when there is just a tiny function in small file you
want to test? The @testable import approach seems to include everything
in the app including the the kitchen sink.