`src/gtirb_pprinter/CMakeLists.txt` builds tests regardless of `GTIRB_PPRINTER_ENABLE_TESTS`

[shamefulCake1]

gtirb-pprinter/src/gtirb_pprinter/CMakeLists.txt

Line 97 in 8447ac9

add_subdirectory(driver)

includes test unconditionally, regardless of whether GTIRB_PPRINTER_ENABLE_TESTS is set.

Should be very easy to fix.