I'm assuming you're on a recent version of OS X with a blazing fast Net connection. These instructions are based on those given by @AutomatedTester
We begin by checking out and building geckodriver. Note: I've had conflicting suggestions from Mozillians about enabling artifact builds. For now, I've disabled them
hg clone https://hg.mozilla.org/mozilla-central/
cd mozilla-central
./mach bootstrap
# Add this to enable (faster!) artifact builds
#echo "ac_add_options --enable-artifact-builds" >> .mozconfig
echo "ac_add_options --enable-geckodriver" >> .mozconfig
./mach configure
./mach build testing/geckodriver
At this point, geckodriver is meant to build quietly and seamlessly. On my machine it's located at ./obj-x86_64-apple-darwin16.6.0/testing/geckodriver/x86_64-apple-darwin/release/geckodriver. Copy that to "somewhere" on your $PATH. If it doesn't compile, we can cheat...:
cd testing/geckodriver
cargo build
cp target/debug/geckodriver $HOME/bin
I'll assume you have $HOME/bin on your $PATH.
Download Firefox Nightly and stick it somewhere handy. I renamed it to "Firefox" in my /Applications directory because that makes life easy for me.
git clone --depth 1 https://github.com/w3c/web-platform-tests.git
cd web-platform-tests
brew install nss
PATH=/usr/local/opt/nss/bin:$PATH ./wpt run --log-wptreport=FF58.json --binary /Applications/Firefox.app/Contents/MacOS/firefox firefox /webdriver
The WebDriver tests should now run. Well done.