Fix broken testshade_dso

In one of the previous changes to hide symbols, we inadvertently broke
testshade_dso on Linux (at least for the LLVM_STATIC case). It can be
fixed by adding the test_shade symbol to the list of things that aren't
hidden. While I'm in there, make the export.h test a bit more exact,
per the way we do it in OIIO.