rbtdb_test fails on 32-bit

Test fails on i686-linux. The problem seems 100% reproducible. Regression of 9.18.17 -> 9.18.18 (which introduced the test, I hear).

FAIL: rbtdb_test
================

[==========] tests: Running 4 test(s).
[ RUN      ] ownercase
[       OK ] ownercase
[ RUN      ] setownercase
[       OK ] setownercase
[ RUN      ] overmempurge_bigrdata
[       OK ] overmempurge_bigrdata
[ RUN      ] overmempurge_longname
[  ERROR   ] --- isc_mem_inuse(mctx2) < maxcache
[   LINE   ] --- rbtdb_test.c:367: error: Failure!
[  FAILED  ] overmempurge_longname
[==========] tests: 4 test(s) run.
[  PASSED  ] 3 test(s).
[  FAILED  ] tests: 1 test(s), listed below:
[  FAILED  ] overmempurge_longname

 1 FAILED TEST(S)
FAIL rbtdb_test (exit status: 1)