There are a couple cases where a single shader might happen:

- compute shaders
  (only one stage, no inputs and outputs; separable shouldn't matter)
- vertex shaders with transform feedback
  (we want to retain outputs, but transform feedback varyings are
   specified via the API, not the shader - setting SSO fixes this)
- old shader_test files captured before we started adding "SSO ENABLED".

In any case, it seems harmless or beneficial to enable SSO for all
.shader_test files containing a single shader.