Investigate flaky mediumint to bigint test

[Shelnutt2]

The alter table test with multiple inplace alters that changes a mediumint to a bigint is failing in travis ci. This failure can not be preproduced anywhere else but travis ci. It was disabled, but should be investigated.

25b4745

Sample failure: https://travis-ci.org/Shelnutt2/crunch/builds/343135301

exception on rnd_next ./test/t1: capnp/layout.c++, line: ../storage/crunch/src/crunch.cpp:346, exception_line: 2159, type: 0, e.what(): expected boundsCheck(segment, ptr, ref->structRef.wordSize()); Message contained out-of-bounds struct pointer.
2018-02-18 22:32:29 139966787450624 [ERROR] mysqld: Got error -44 "Internal error < 0 (Not system error)" from storage engine Crunch