gdb/testsuite: gdb.base/continue-all-already-running.exp: add fail if can't run to main

While doing some changes, I managed to break this test in a way that
running to main didn't work.  However, it didn't produce any FAIL.  I
noticed because I diff'ed the results and saw some PASSes unexpectedly
disappear, but that's a bit fragile.  Add a fail in case this test fails
to run to main.  Ideally, I think that runto_main should by default
produce a FAIL when it fails (the opposite of the existing logic), but
that's a project of its own, so just fix this test for the moment.

gdb/testsuite/ChangeLog:

	* gdb.base/continue-all-already-running.exp: Call fail if can't
	run to main.

Change-Id: I84b816a126c92ac579ed5ebbe39b017bd5cb7096

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-06-07  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* gdb.base/continue-all-already-running.exp: Call fail if can't
+	run to main.
+
 2021-06-07  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	PR gdb/27847

gdb/testsuite/gdb.base/continue-all-already-running.exp

@@ -26,6 +26,7 @@ save_vars { GDBFLAGS } {
 }
 
 if ![runto_main] {
+    fail "can't run to main"
     return
 }