Increase data rlimit to 8GB.

mandelbrot_mutable and index_overflow_test2 seem to run into this limit
and crash with "SIGSEGV: Resource temporarily unavailable".

mjtest/util/shell.py

@@ -58,7 +58,7 @@ class _Execute(object):
         if not 'RLIMIT_CORE' in rlimit:
             rlimit['RLIMIT_CORE'] = 0
         if not 'RLIMIT_DATA' in rlimit:
-            rlimit['RLIMIT_DATA'] = 1024 * MB
+            rlimit['RLIMIT_DATA'] = 8192 * MB
         if not 'RLIMIT_STACK' in rlimit:
             rlimit['RLIMIT_STACK'] = 1024 * MB
         if not 'RLIMIT_FSIZE' in rlimit: