travis: put clar's sandbox in a ramdisk on macOS

The macOS tests are by far the slowest right now. This attempts to remedy the
situation somewhat by asking clar to put its test data on a ramdisk.

(cherry picked from commit 37bb1512)

script/cibuild.sh

@@ -10,6 +10,15 @@ fi
 
 if [ "$TRAVIS_OS_NAME" = "osx" ]; then
 	export PKG_CONFIG_PATH=$(ls -d /usr/local/Cellar/{curl,zlib}/*/lib/pkgconfig | paste -s -d':' -)
+
+	# Set up a ramdisk for us to put our test data on to speed up tests on macOS
+	export CLAR_TMP="$HOME"/_clar_tmp
+	mkdir -p $CLAR_TMP
+
+	# 2M sectors aka ~1GB of space
+	device=$(hdiutil attach -nomount ram://$((2 * 1024 * 1024)))
+	newfs_hfs $device
+	mount -t hfs $device $CLAR_TMP
 fi
 
 # Should we ask Travis to cache this file?