Random test failures in t/re-sub.t

[xiaocang]

I'm seeing the following random test failures on Fedora x86_64:

$ TEST_NGINX_INIT_BY_LUA="jit.opt.start('minstitch=0') require('resty.core.base').set_string_buf_size(1) require('resty.core.regex').set_buf_grow_ratio(1)" \
    TEST_NGINX_SLEEP=0.005 \
	prove -r -I../test-nginx/lib t/re-sub.t

t/re-sub.t .. 1/106 
#   Failed test 'TEST 4: replace template + submatches - pattern "NYI" should not match any line in error.log but matches line "[TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]" (req 0
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE   3 (1/0) base.lua:184 -- fallback to interpreter]
# [TRACE --- regex.lua:1189 -- inner loop in root trace at regex.lua:1082]
# [TRACE --- (2/0) regex.lua:829 -- loop unroll limit reached at regex.lua:1199]
# [TRACE   4 regex.lua:348 return]
# [TRACE   5 regex.lua:814 return]
# [TRACE --- (2/0) regex.lua:829 -- loop unroll limit reached at regex.lua:1199]
# '
#   at /home/dev/work/org/lua-resty-core/../test-nginx/lib/Test/Nginx/Socket.pm line 1280.

t/re-sub.t .. 35/106 
#   Failed test 'TEST 1: sub, no submatch, no jit compile, regex cache - pattern "NYI" should not match any line in error.log but matches line "[TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at 
ffi.new]" (req 0)
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE --- (1/0) base.lua:184 -- NYI: return to lower frame at ffi.new]
# [TRACE --- regex.lua:1060 -- inner loop in root trace at regex.lua:1082]
# [TRACE   3 (1/0) base.lua:184 -- fallback to interpreter]
# [TRACE --- (2/0) regex.lua:829 -- loop unroll limit reached at regex.lua:1204]
# [TRACE --- regex.lua:1189 -- inner loop in root trace at regex.lua:1082]
# [TRACE --- (2/0) regex.lua:829 -- loop unroll limit reached at regex.lua:1204]
# [TRACE --- (2/0) regex.lua:829 -- loop unroll limit reached at regex.lua:1204]
# '
#   at /home/dev/work/org/lua-resty-core/../test-nginx/lib/Test/Nginx/Socket.pm line 1280.