gdb: Fix DUPLICATE and PATH regressions throughout

gdb: Fix DUPLICATE and PATH regressions throughout

The previous patch to add -prompt/-lbl to gdb_test introduced a
regression: Before, you could specify an explicit empty message to
indicate you didn't want to PASS, like so:

  gdb_test COMMAND PATTERN ""

After said patch, gdb_test no longer distinguishes
no-message-specified vs empty-message, so tests that previously would
be silent on PASS, now started emitting PASS messages based on
COMMAND.  This in turn introduced a number of PATH/DUPLICATE
violations in the testsuite.

This commit fixes all the regressions I could see.

This patch uses the new -nopass feature introduced in the previous
commit, but tries to avoid it if possible.  Most of the patch fixes
DUPLICATE issues the usual way, of using with_test_prefix or explicit
unique messages.

See previous commit's log for more info.

In addition to looking for DUPLICATEs, I also looked for cases where
we would now end up with an empty message in gdb.sum, due to a
gdb_test being passed both no message and empty command.  E.g., this
in gdb.ada/bp_reset.exp:

 gdb_run_cmd
 gdb_test "" "Breakpoint $decimal, foo\\.nested_sub \\(\\).*"

was resulting in this in gdb.sum:

 PASS: gdb.ada/bp_reset.exp:

I fixed such cases by passing an explicit message.  We may want to
make such cases error out.

Tested on x86_64 GNU/Linux, native and native-extended-gdbserver.  I
see zero PATH cases now.  I get zero DUPLICATEs with native testing
now.  I still see some DUPLICATEs with native-extended-gdbserver, but
those were preexisting, unrelated to the gdb_test change.

Change-Id: I5375f23f073493e0672190a0ec2e847938a580b2