[gdb/testsuite] Fix py-autoloaded-pretty-printers-in-newobjfile-event.exp for remote target

With test-case gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp
and target board remote-gdbserver-on-localhost, I run into:
...
FAIL: $exp: runto: run to main
...

I can easily fix this using "gdb_load_shlib $binfile_lib", but then run into:
...
(gdb) print all_good^M
$1 = false^M
(gdb) FAIL: $exp: print all_good
info pretty-printer^M
...

Sysroot is set to "target:", so gdb downloads the shared library from the target
(Using $so as shorthand for
libpy-autoloaded-pretty-printers-in-newobjfile-event.so):
...
Reading /home/remote-target/$so from remote target...^M
...
and internally refers to it as "target:/home/remote-target/$so".

In load_auto_scripts_for_objfile, gdb gives up trying to auto-load scripts
for $so once it checks for is_target_filename.

Fix this by declaring auto-load unsupported if sysroot starts with "target:".

Tested on x86_64-linux.