Fix PR testsuite/1729.
* gdb.base/dump.exp: Add test name to callers of capture_value, do
not use capture_value with value 4.
(capture_value): Add optional test name parameter.
(test_reload_saved_value, test_restore_saved_value): Use $msg as
the prefix, pass to capture_value.
* gdb.base/default.exp: Add tests for dump, append, and restore.
* gdb.base/help.exp: Add tests for dump, append, and restore.
* gdb.base/dump.exp: New file, test dump, append and restore.
* gdb.base/dump.c: New file.