test/gen_testdata: Prevent object with OID=1 to be deleted

Previously it was like this by luck, but recent changes shuffled
generated output and one of delete selection started to point to OID 1:

    (zdev+py39.env) kirr@deca:~/src/wendelin/z/zodbtools/zodbtools/test/testdata$ grep '^obj .* delete' 1.zdump.zpickledis.ok
    obj 0000000000000001 delete
    obj 0000000000000002 delete

However NEO/go tests depend on object with OID=1 to be present:

https://lab.nexedi.com/kirr/neo/-/blob/1ad088c8/go/neo/neo_test.go#L216-219

and it breaks there if here we let that object with OID=1 to be deleted.

-> Prevent ZODB/go breakage by explicitly making sure that OID=1 stays
present under any conditions.

The actual places where gen_testdata.py from zodbtools is used in
ZODB/go and Wendelin.core are e.g. here:

https://lab.nexedi.com/kirr/neo/-/blob/f7776fc1/go/zodb/storage/fs1/py/gen-testdata
https://lab.nexedi.com/nexedi/wendelin.core/-/blob/07087ec8/wcfs/internal/zdata/testdata/zblk_test_gen.py

zodbtools/test/gen_testdata.py

@@ -37,6 +37,8 @@
 # NOTE as of 14 Mar 2017 FileStorage cannot commit transactions with non-ASCII
 #      metadata - so it is not tested
 
+# NOTE besides zodbtools this module is also used in ZODB/go and in Wendelin.core .
+
 from ZODB.FileStorage import FileStorage
 from ZODB import DB
 from ZODB.Connection import TransactionMetaData
@@ -216,6 +218,7 @@ def _gen_testdb(outfs_path, zext):
 
     namev = [_ for _ in "abcdefg"]
     Niter = 3
+    nameobj1 = None  # name used when adding first object
     for i in range(Niter):
         stor = FileStorage(outfs_path, create=(i == 0))
         db   = DB(stor)
@@ -238,6 +241,10 @@ def _gen_testdb(outfs_path, zext):
 
             commit(u"user%i.%i" % (i,j), u"step %i.%i" % (i, j), ext(name))
 
+            if nameobj1 is None:
+                nameobj1 = name
+                assert obj._p_oid == p64(1), repr(obj._p_oid)
+
         # undo a transaction one step before a latest one a couple of times
         for j in range(2):
             # XXX undoLog, despite what its interface says:
@@ -267,7 +274,8 @@ def _gen_testdb(outfs_path, zext):
         commit(u"user", u"cyclic reference", ext("cycle"))
 
         # delete an object
-        name = rand.choice(keys(root))
+        _ = keys(root);  _.remove(nameobj1)  # preserve the first obj not to go
+        name = rand.choice(_)
         obj = root[name]
         root[name] = Object("%s%i*" % (name, i))
         # NOTE user/ext are kept empty on purpose - to also test this case