Background:

Cmake creates ${cmake_binary_dir}/sql/share/ regardless of
INSTALL_SHAREDIR setting, to "fix" this MTR sets --lc-messages-dir and
--basedir options.

mysqld in main.basedir runs without --basedir and --lc-messages-dir
options and breaks due to mismatch between compiled-in install layout
and sandbox layout.

Solution:

Make both source and build sandbox layout more identical to install
layout by moving sql/share/ to share/, such that
runtime_output_directory/ and share/ will have common parent
(${cmake_binary_dir}) like in install layout.

This make sense since WL#10441 as mysqld will find basedir and
lc-messages-dir itself, if cmake creates share directory in
${cmake_binary_dir}/ according to -DINSTALL_MYSQLSHAREDIR, things
would work out of box without options specified. This will work in
build sandbox and after "make install". In other words mysqld basedir
is moved from ${cmake_binary_dir}/sql/ to ${cmake_binary_dir}/ in
sandbox.

After change options --basedir and --lc-messages-dir can be removed
from MTR runs, and special cases in mysqld.cc can be removed too.
Search path "sql/share" is removed or adjusted.

Tests fragments that don't have stable output any longer have been
removed.

Basedir option is used by MTR to find "itself" in sandbox mode, hence
it can't be removed even if not used by mysqld. Variable is renamed
to #mtr_basedir in generated my.cnf.

Note:

This means errmsg-utf8.txt is moved from sql/share to share/,
take care when merging from older major versions.