Increase heap size for bwc tests

Saw some GC warnings in the logs:
```
[o.e.m.j.JvmGcMonitorService] [0faf0d8D1BBa-2] [gc][6] overhead, spent [413ms] collecting in the last [1.1s]
```

High GC could explain issues during cluster formation and the timeout
errors.

Author: mfussenegger

src/crate/qa/tests.py

@@ -167,7 +167,7 @@ def __getitem__(self, idx):
 class NodeProvider:
 
     CRATE_VERSION = os.environ.get('CRATE_VERSION', 'latest-nightly')
-    CRATE_HEAP_SIZE = os.environ.get('CRATE_HEAP_SIZE', '512m')
+    CRATE_HEAP_SIZE = os.environ.get('CRATE_HEAP_SIZE', '1024m')
     DEBUG = os.environ.get('DEBUG', 'false').lower() == 'true'
 
     def __init__(self, *args, **kwargs):