That was my first idea but then I thought there's a lot of hacking to prepare such param list with msbuild. It happened to be easier than I thought and the following syntax worked perfectly:
<ItemGroup>
<TestSpecs Include="**\*Spec.js" />
</ItemGroup>
<Target Name="Test" DependsOnTargets="PackageRestore">
<MakeDir Directories="$(TestResultsDir)"/>
<Exec Command="$(ChutzpahExePath) @(TestSpecs->'/path %(FullPath)', ' ') /junit $(TestResultsDir)/js-test-results.junit.xml /coverage" IgnoreExitCode="true"/>
</Target>
Thanks for straightening me out on this.