Fix regression where test targets don’t have anything to build on executable-producing projects.

[nicholatian]

When compiling the cov, asan and ubsan targets on library code, Slick is supposed to defer to TES batteries. If there are no batteries, there is nothing to build.

With projects that produce standalone executables, this is not the approach. Instead, Slick will build the executable as normal and instrument it for coverage, A-san or UB-san as requested.

This regression happened since TES support was rebuilt after having rusted during the 1.3 release series, and was overlooked as at the time I was dogfooding 1.4 stuff using library code. It should be an easy fix.