tox: Add `auto` block format to our test matrix

nexedi/wendelin.core!20 (comment 198341)

tox.ini

 # wendelin.core | tox setup
 [tox]
-envlist = py27-{ZODB4,ZODB5}-{zblk0,zblk1}-{fs,zeo,neo}-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
-          {py36,py37}-{ZODB4,ZODB5}-{zblk0,zblk1}-fs-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
-          py36-{ZODB4,ZODB5}-{zblk0,zblk1}-zeo-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
-          py37-ZODB5-{zblk0,zblk1}-zeo-{numpy115,numpy116-{!wcfs,wcfs,wcfs:1,wcfs:2}}
+envlist = py27-{ZODB4,ZODB5}-{zblk0,zblk1,auto}-{fs,zeo,neo}-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
+          {py36,py37}-{ZODB4,ZODB5}-{zblk0,zblk1,auto}-fs-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
+          py36-{ZODB4,ZODB5}-{zblk0,zblk1,auto}-zeo-{numpy115,numpy116}-{!wcfs,wcfs,wcfs:1,wcfs:2},
+          py37-ZODB5-{zblk0,zblk1,auto}-zeo-{numpy115,numpy116-{!wcfs,wcfs,wcfs:1,wcfs:2}}
 # (NOTE ZEO4 does not work with python3.7)
 # (NOTE NEO does not work on python3 at all)
 # (XXX ZODB5-*-neo are currently failing)
@@ -40,6 +40,7 @@ setenv =
 
     zblk0:  WENDELIN_CORE_ZBLK_FMT=ZBlk0
     zblk1:  WENDELIN_CORE_ZBLK_FMT=ZBlk1
+    auto:   WENDELIN_CORE_ZBLK_FMT=auto
 
     !wcfs:  WENDELIN_CORE_VIRTMEM=rw:uvmm
     wcfs:   WENDELIN_CORE_VIRTMEM=r:wcfs+w:uvmm