gdb/testsuite: make runto_main not pass no-message to runto

As follow-up to this discussion:

  https://sourceware.org/pipermail/gdb-patches/2020-August/171385.html

... make runto_main not pass no-message to runto.  This means that if we
fail to run to main, for some reason, we'll emit a FAIL.  This is the
behavior we want the majority of (if not all) the time.

Without this, we rely on tests logging a failure if runto_main fails,
otherwise.  They do so in a very inconsisteny mannet, sometimes using
"fail", "unsupported" or "untested".  The messages also vary widly.
This patch removes all these messages as well.

Also, remove a few "fail" where we call runto (and not runto_main).  by
default (without an explicit no-message argument), runto prints a
failure already.  In two places, gdb.multi/multi-re-run.exp and
gdb.python/py-pp-registration.exp, remove "message" passed to runto.
This removes a few PASSes that we don't care about (but FAILs will still
be printed if we fail to run to where we want to).  This aligns their
behavior with the rest of the testsuite.

Change-Id: Ib763c98c5f4fb6898886b635210d7c34bd4b9023

gdb/testsuite/gdb.python/py-block.exp

@@ -28,7 +28,6 @@ if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
 if { [skip_python_tests] } { continue }
 
 if ![runto_main] then {
-    fail "can't run to main"
     return 0
 }