gdb: fix gdb.gdb/unittest.exp with C++17 compiler

gdb: fix gdb.gdb/unittest.exp with C++17 compiler

On a machine with gcc 11, I get:

    FAIL: gdb.gdb/unittest.exp: test_completion: tab complete "maintenance selftest string_v" (second tab) (timeout)
    FAIL: gdb.gdb/unittest.exp: test_completion: tab complete "maintenance selftest string_vie" (timeout)

That's because when compiling with C++ >= 17, we use the standard
version of string_view, and don't have a selftest for it.  So the list
of selftests shown by the tab completion when completing "string_v"
differs.

Change the test to use the copy_* tests instead.

Change-Id: I85f6aa44ee5fc9652b9bd4451e0506b89773526b