gdb/testsuite: make gdb_unload use gdb_test_multiple

gdb/testsuite: make gdb_unload use gdb_test_multiple

In the failure seen by Philippe here:

  https://inbox.sourceware.org/gdb-patches/20221120173024.3647464-1-philippe.waroquiers@skynet.be/

gdb_unload crashed GDB, leaving no trace in the test results.  Change it
to use gdb_test_multiple, so that it leaves an UNRESOLVED result.  I
think it is good practice anyway.

Make it return the result of gdb_test_multiple directly, change
gdb.python/py-objfile.exp accordingly.

Change gdb.base/endian.exp as well to avoid duplicate test names.

Change gdb.base/gnu-debugdata.exp to avoid recording a test result,
since gdb_unload does it already now.

Change-Id: I59a1e4947691330797e6ce23277942547c437a48
Approved-By: Tom de Vries <tdevries@suse.de>