"IOError: Can't find a path to system files" for x86 full system on Mac - gem5

Question

I'm trying to set up gem5 x86 full system on Mac OS Mojave (10.14)
First I did a git clone to get the gem5 sources, which are located at ~/gem5.
Then I ran scons build/x86/gem5.fast to build the whole thing. I had to change some of the -Werror flags to get it to compile, but it seems to work.
To test it, I ran build/x86/gem5.fast configs/example/se.py -c tests/test-progs/hello/bin/x86/linux/hello and got the following output:
gem5 Simulator System. http://gem5.org
gem5 is copyrighted software; use the --copyright option for details.
gem5 compiled Jan 18 2020 15:28:50
gem5 started Jan 18 2020 17:48:35
gem5 executing on My-MacBook-Pro-208.local, pid 89984
command line: build/x86/gem5.fast configs/example/se.py -c tests/test-progs/hello/bin/x86/linux/hello
Global frequency set at 1000000000000 ticks per second
warn: DRAM device capacity (8192 Mbytes) does not match the address range assigned (512 Mbytes)
0: system.remote_gdb: listening for remote gdb on port 7000
**** REAL SIMULATION ****
info: Entering event queue # 0. Starting simulation...
Hello world!
Exiting # tick 5941500 because exiting with last active thread context
I wanted to configure full system simulation, so I went to the "Full-System Stuff" section at http://gem5.org/Download and downloaded the Full System Files. I extracted the tar into ~/gem5/x86-system.
So now there's ~/gem5/x86-system/binaries which contains x86_64-vmlinux-2.6.22.9 and ~/gem5/x86-system/disks which contains linux-x86.img
In ~/.bash_profile I added export M5_PATH="/Users/me/gem5/x86-system".
However, when I run scons build/x86/tests/fast/quick, almost all of the tests fail. A lot of them have a failure like this:
...
File "/Users/me/gem5/configs/common/SysPaths.py", line 62, in __call__
raise IOError("Can't find a path to system files.")
IOError: Can't find a path to system files.
I also tried to run build/x86/gem5.fast configs/example/fs.py but I get the following error:
...
File "/Users/me/gem5/configs/common/SysPaths.py", line 71, in __call__
raise IOError("Can't find file '%s' on path." % filename)
IOError: Can't find file 'x86root.img' on path.
I'm not sure what part of configuration I'm missing. The docs and google searches aren't giving any working solutions...