The function alloc_and_copy_thd_dynamic_variables() synchronizes
session variables with global variables. As of Bug#21840950, it uses
the mutex THD::LOCK_thd_sysvar to block remote threads from accessing
session vars during updates.

With SHOW_COMPATIBILITY_56=OFF, SHOW VARIABLES can cause
LOCK_thd_sysvar to be taken twice, first in System_variable::init()
and later in alloc_and_copy_thd_dynamic_variables().

System_variables::init() now only acquires LOCK_thd_sysvar if the
target thread is not the current thread.

perfschema.show_plugin.test is now two tests so that
alloc_and_copy_thd_dynamic_variables() can be tested separately
with SHOW_COMPATIBILITY_56 = ON and OFF.

(cherry picked from commit 8d91476c0b2bc510729a786ab3ba98ef6d28c2aa)