Error when running dmd compilable tests

[Per Nordlöw]

Inside dmd source tree, under sub-directory test, I do

     ./run.d compilable/

. This errors as

```
  ... compilable/json.d              -d -preview=dip1000 -o- -X 
-Xf/home/per/Work/dmd/test/test_results/compilable/json.out  
-fPIC ()==============================
Test 'compilable/json.d' failed. The logged output:
Executing post-test script: tools/postscript.sh 
compilable/extra-files/json-postscript.sh compilable json 
/home/per/Work/dmd/test/test_results/compilable/json.d.out0
tools/postscript.sh: line 18: /home/per/Work/dmd/test/tools
/home/per/Work/dmd/test/tools/exported_vars.sh: No such file or 
directory
/home/per/Work/dmd/generated/linux/release/64/dmd -conf= -m64 
-Icompilable -d -preview=dip1000 -o- -X 
-Xf/home/per/Work/dmd/test/test_results/compilable/json.out  
-fPIC  -od/home/per/Work/dmd/test/test_results/compilable 
-of/home/per/Work/dmd/test/test_results/compilable/json_0.o  -c 
compilable/json.d

==============================
Test 'compilable/json.d' failed: Expected rc == 0, but exited 
with rc == 1

>>> TARGET FAILED: compilable/json.d
  ... compilable/ddoc8.d             -D 
-Dd/home/per/Work/dmd/test/test_results/compilable -o-  -fPIC 
()==============================
```

What's wrong?