Adjust the tests in the json test suite so that they run cleanly
regardless of what the default character set of the test db is (latin1
or utf8mb4).

Changes:

- json_client.inc: Create the test table with explicit character set,
  so that the table metadata are unchanged when the default character
  set is changed.

- json_fulltext.inc: Remove a SHOW CREATE TABLE statement that is not
  essential for the test. Its output varies depending on what the
  default character set is.

- json_functions.inc: Remove a redundant SHOW CREATE TABLE statement
  (the same statement is executed on an identical table in
  json_insert.inc). Also use --sorted_result instead of ORDER BY in
  some statements where the ordering differs depending on character
  set ('1' sorts before '[' with latin1, but in the opposite order
  with utf8mb4).

- json_group_concat.inc: Use explicit character set in a CREATE TABLE
  statement to ensure consistent ordering (the ordering of some
  control characters is different in latin1 and utf8mb4).

- json_insert.inc: Use explicit character set in a CREATE TABLE
  statement to make the output from SHOW CREATE TABLE independent of
  the default character set.

- json_agg.test: Use explicit character set in a CREATE TABLE
  statement to make JSON-formatted explain output independent of the
  default character set.