With the growing number of selftests, I think it would be useful to be
able to run only a subset of the tests.  This patch associates a name to
each registered selftest.  It then allows doing something like:

  (gdb) maintenance selftest aarch64
  Running self-tests.
  Running selftest aarch64-analyze-prologue.
  Running selftest aarch64-process-record.
  Ran 2 unit tests, 0 failed

or with gdbserver:

  ./gdbserver --selftest=aarch64

In both cases, only the tests that contain "aarch64" in their name are
ran.  To help validate that the tests you want to run were actually ran,
it also prints a message with the test name before running each test.

Right now, all the arch-dependent tests are registered as a single test
of the selftests.  To be able to filter those too, I made them
"first-class citizen" selftests.  The selftest type is an interface,
with different implementations for "simple selftests" and "arch
selftests".  The run_tests function simply iterates on that an invokes
operator() on each test.

I changed the tests data structure from a vector to a map, because

  - it allows iterating in a stable (alphabetical) order
  - it allows to easily verify if a test with a given name has been
    registered, to avoid duplicates

There's also a new command "maintenance info selftests" that lists the
registered selftests.

gdb/ChangeLog:

	* common/selftest.h (selftest): New struct/interface.
	(register_test): Add name parameter, add new overload.
	(run_tests): Add filter parameter.
	(for_each_selftest_ftype): New typedef.
	(for_each_selftest): New declaration.
	* common/selftest.c (tests): Change type to
	map<string, unique_ptr<selftest>>.
	(simple_selftest): New struct.
	(register_test): New function.
	(register_test): Add name parameter and use it.
	(run_tests): Add filter parameter and use it.  Add prints.
	Adjust to vector -> map change.
	* aarch64-tdep.c (_initialize_aarch64_tdep): Add names when
	registering selftests.
	* arm-tdep.c (_initialize_arm_tdep): Likewise.
	* disasm-selftests.c (_initialize_disasm_selftests): Likewise.
	* dwarf2-frame.c (_initialize_dwarf2_frame): Likewise.
	* dwarf2loc.c (_initialize_dwarf2loc): Likewise.
	* findvar.c (_initialize_findvar): Likewise.
	* gdbarch-selftests.c (_initialize_gdbarch_selftests): Likewise.
	* maint.c (maintenance_selftest): Update call to run_tests.
	(maintenance_info_selftests): New function.
	(_initialize_maint_cmds): Register "maintenance info selftests"
	command.  Update "maintenance selftest" doc.
	* regcache.c (_initialize_regcache): Add names when registering
	selftests.
	* rust-exp.y (_initialize_rust_exp): Likewise.
	* selftest-arch.c (gdbarch_selftest): New struct.
	(gdbarch_tests): Remove.
	(register_test_foreach_arch): Add name parameter.  Call
	register_test.
	(tests_with_arch): Remove, move most content to
	gdbarch_selftest::operator().
	(_initialize_selftests_foreach_arch): Remove.
	* selftest-arch.h (register_test_foreach_arch): Add name
	parameter.
	(run_tests_with_arch): New declaration.
	* utils-selftests.c (_initialize_utils_selftests): Add names
	when registering selftests.
	* utils.c (_initialize_utils): Likewise.
	* unittests/array-view-selftests.c
	(_initialize_array_view_selftests): Likewise.
	* unittests/environ-selftests.c (_initialize_environ_selftests):
	Likewise.
	* unittests/function-view-selftests.c
	(_initialize_function_view_selftests): Likewise.
	* unittests/offset-type-selftests.c
	(_initialize_offset_type_selftests): Likewise.
	* unittests/optional-selftests.c
	(_initialize_optional_selftests): Likewise.
	* unittests/scoped_restore-selftests.c
	(_initialize_scoped_restore_selftests): Likewise.
	* NEWS: Document "maintenance selftest" and "maint info
	selftests".

gdb/gdbserver/ChangeLog:

	* server.c (captured_main): Accept argument for --selftest.
	Update run_tests call.
	* linux-x86-tdesc-selftest.c (initialize_low_tdesc): Add names
	when registering selftests.

gdb/doc/ChangeLog:

	* gdb.texinfo (Maintenance Commands): Document filter parameter
	of "maint selftest".  Document "maint info selftests" command.