ci: run v self with -prealloc

.github/workflows/ci.yml

@@ -62,6 +62,8 @@ jobs:
           make
           ./v -d debug_malloc -d debug_realloc -o v cmd/v
           ./v -cg -cflags -Werror -o v cmd/v
+          # Test v -realloc arena allocation
+          ./v -o vrealloc -prealloc cmd/v && ./vrealloc -o v3 cmd/v && ./v3 -o v4 cmd/v
       - name: Test v->c
         run: |
           thirdparty/tcc/tcc.exe -version