build: Don't try to run tests if there is no yosys
build: Don't try to run tests if there is no yosys

Fixes #80.

Also fixes BUILD_DOC (for real) when pdflatex is missing

azonenberg/openfpga,rqou/openfpga,rqou/openfpga,azonenberg/openfpga,rqou/openfpga,rqou/openfpga,azonenberg/openfpga,rqou/openfpga,azonenberg/openfpga,azonenberg/openfpga