Look for a test asset, and if it exists, copy it to the
test context at the given location.
function copyAssetToContext(src, dest, context) {
    var sourceAsset = path.join(testDir, src);
    var targetAsset = path.join(context.dir, dest);

    if (!fs.existsSync(sourceAsset)) {
        // The source asset requested is missing
        throw new Error("The test asset is missing");
    }

    // Attempt to copy the asset
    fs.copySync(sourceAsset, targetAsset);

    if (!fs.existsSync(targetAsset)) {
        // The target asset was not copied successfully
        throw new Error("The test asset could not be copied");
    }
}