pytest: Fix optimistic locking in valgrind - developer config

Valgrind doesn't really like crashes if compiled without DEVELOPER since that
seems to compile out the debug symbols, resulting in the following error:

```
Optimistic lock on the database failed. There may be a concurrent access to the database. Aborting since concurrent access is unsafe.
lightningd: FATAL SIGNAL 6 (version 0.0.99)
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd: FATAL SIGNAL 11 (version 0.0.99)
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
lightningd/lightningd: libbacktrace: no debug info in ELF executable
2020-01-07 15:26:03.539 EST [11583] LOG:  unexpected EOF on client connection with an open transaction
--------------------------- Captured stdout teardown ---------------------------
DEBUG:root:Calling stop with payload None
------------------------------- Valgrind errors --------------------------------
Valgrind error file: valgrind-errors.11409
==11409== Jump to the invalid address stated on the next line
==11409==    at 0x0: ???
==11409==    by 0x1C00A8: backtrace_full (backtrace.c:127)
==11409==    by 0x147B0A: send_backtrace (daemon.c:46)
==11409==    by 0x147B55: crashdump (daemon.c:54)
==11409==    by 0x6071F1F: ??? (in /lib/x86_64-linux-gnu/libc-2.27.so)
==11409==    by 0x6071E96: __libc_signal_restore_set (nptl-signals.h:80)
==11409==    by 0x6071E96: raise (raise.c:48)
==11409==    by 0x6073800: abort (abort.c:79)
==11409==    by 0x12B2FF: fatal (log.c:819)
==11409==    by 0x16FA3B: db_data_version_incr (db.c:826)
==11409==    by 0x16FA9E: db_commit_transaction (db.c:841)
==11409==    by 0x124D20: io_loop_with_timers (io_loop_with_timers.c:34)
==11409==    by 0x129260: main (lightningd.c:860)
==11409==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==11409==
==11409==
==11409== Process terminating with default action of signal 11 (SIGSEGV)
==11409==  Bad permissions for mapped region at address 0x0
==11409==    at 0x0: ???
==11409==    by 0x1C00A8: backtrace_full (backtrace.c:127)
==11409==    by 0x147B0A: send_backtrace (daemon.c:46)
==11409==    by 0x147B55: crashdump (daemon.c:54)
==11409==    by 0x6071F1F: ??? (in /lib/x86_64-linux-gnu/libc-2.27.so)
--------------------------------------------------------------------------------
```

tests/test_db.py

@@ -1,7 +1,7 @@
 from fixtures import *  # noqa: F401,F403
 from fixtures import TEST_NETWORK
 from pyln.client import RpcError
-from utils import wait_for, sync_blockheight, COMPAT
+from utils import wait_for, sync_blockheight, COMPAT, VALGRIND, DEVELOPER
 import os
 import pytest
 import time
@@ -140,6 +140,7 @@ def test_scid_upgrade(node_factory, bitcoind):
     assert l1.db_query('SELECT failchannel from payments;') == [{'failchannel': '103x1x1'}]
 
 
+@unittest.skipIf(VALGRIND and not DEVELOPER, "Without developer valgrind will complain about debug symbols missing")
 def test_optimistic_locking(node_factory, bitcoind):
     """Have a node run against a DB, then change it under its feet, crashing it.