New set/show testing framework (gdb.base/settings.exp)

New set/show testing framework (gdb.base/settings.exp)

This commit adds new representative commands for all types of settings
commands supported by gdb (enum var_types), and then uses them to
exercise settings parsing and completion.

  (gdb) maint test-settings s[TAB]
  set   show

  (gdb) maint test-settings set [TAB]
  auto-boolean         integer              uinteger
  boolean              optional-filename    zinteger
  enum                 string               zuinteger
  filename             string-noescape      zuinteger-unlimited

  (gdb) maint test-settings set enum [TAB]
  xxx  yyy  zzz

  etc.

This is basically unit testing, except that it goes fully via GDB.  It
must be done this way in order to exercise TAB completion properly,
which must go via readline.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_SFILES): Add maint-test-settings.c.
	* NEWS: Mention maint test-settings KIND.
	* maint-test-settings.c: New file.

gdb/doc/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Maintenance Commands): Document "maint
	test-settings" commands.

gdb/testsuite/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/settings.c: New file.
	* gdb.base/settings.exp: New file.