test: increase quota for tests to 6GB

test.py estimates the amount of memory needed per test
in order not to overload the machine, but it underestimates
badly and so machines with many cores but not a lot of memory
fail the tests (in debug mode principally) due to running out
of memory.

Increase the estimate from 2GB per test to 6GB.

Closes #7499

test.py

@@ -509,7 +509,7 @@ def parse_cmd_line():
     """ Print usage and process command line options. """
     all_modes = ['debug', 'release', 'dev', 'sanitize']
     sysmem = os.sysconf('SC_PAGE_SIZE') * os.sysconf('SC_PHYS_PAGES')
-    testmem = 2e9
+    testmem = 6e9
     cpus_per_test_job = 1
     default_num_jobs_mem = ((sysmem - 4e9) // testmem)
     default_num_jobs_cpu = multiprocessing.cpu_count() // cpus_per_test_job