pytest: don't assume paths of valgrind and bitcoind.

In particular, my bitcoind is in /usr/local/bin

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/utils.py

@@ -133,7 +133,7 @@ class BitcoinD(TailableProc):
             os.makedirs(regtestdir)
 
         self.cmd_line = [
-            '/usr/bin/bitcoind',
+            'bitcoind',
             '-datadir={}'.format(bitcoin_dir),
             '-printtoconsole',
             '-server',