tests.replaceVars: fix group mismatches
This fixes group mismatches observed on Darwin, e.g.:
```
replaceVars-succeeds
--- /nix/store/wahj58fiii5j1jy6s9mpbdyxzhq3plnb-source.txt
+++ /nix/store/bfmykrc9r5ycy0dj8l1z2p9c5116dny8-expected
├── stat {}
│ @@ -1,7 +1,7 @@
│
│ Size: 96 Blocks: 0 IO Block: 4096 directory
│ -Device: 1,15 Access: (0555/dr-xr-xr-x) Uid: ( 0/ root) Gid: ( 0/ wheel)
│ +Device: 1,15 Access: (0555/dr-xr-xr-x) Uid: ( 0/ root) Gid: ( 350/ nixbld)
│
│ Modify: 1970-01-01 00:00:01.000000000 +0000
```
Michael Daniels
3a5c240ffe