Fix some warnings

Summary:
On GCC, my effective CATCH_ERRORS config is:

add_definitions("-Werror -Wall -Wextra -Wno-unknown-pragmas -Wno-unused-function -Wno-cast-align -Wno-shadow -Wno-deprecated -Wno-old-style-cast -Wno-unused-parameter -Wno-padded -Wno-missing-noreturn -Wno-sign-conversion -Wno-vla -Wno-zero-as-null-pointer-constant -Wno-switch-enum -Wno-unused-result -Wno-unused-value")

• -Wno-unused-result because of the calls to the external program populatemailbox.sh and such
• -Wno-unused-value because apparently the call to the VERIFYEXEC_RET macro in imapmailsynctest.cpp in the createMessage function has a statement that does nothing

Reviewers: cmollekopf

Tags: Sink

Differential Revision: https://phabricator.kde.org/D15057