gdb: fix for gdb.base/eof-exit.exp test failures

This fix relates to PR gdb/29032, this makes the test more stable by
ensuring that the Ctrl-D is only sent once the prompt has been
displayed.  This issue was also discussed on the mailing list here:

  https://sourceware.org/pipermail/gdb-patches/2022-April/187670.html

The problem identified in the bug report is that sometimes the Ctrl-D
(that the test sends to GDB) arrives while GDB is processing a
command.  When this happens the Ctrl-D is handled differently than if
the Ctrl-D is sent while GDB is waiting for input at a prompt.

The original intent of the test was that the Ctrl-D be sent while GDB
was waiting at a prompt, and that is the case the occurs most often,
but, when the Ctrl-D arrives during command processing, then GDB will
ignore the Ctrl-D, and the test will fail.

This commit ensures the Ctrl-D is always sent while GDB is waiting at
a prompt, which makes this test stable.

But, that still leaves an open question, what should happen when the
Ctrl-D arrives while GDB is processing a command?  This commit doesn't
attempt to answer that question, which is while bug PR gdb/29032 will
not be closed once this commit is merged.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29032

gdb/testsuite/gdb.base/eof-exit.exp

@@ -25,9 +25,27 @@ proc run_test {} {
     #
     # Send a newline character, which will cause GDB to redisplay the
     # prompt.
+    #
+    # We then consume the newline characters, and then make use of
+    # expect's -notransfer option to ensure that the prompt has been
+    # displayed, but to leave the prompt in expect's internal buffer.
+    # This is important as the following test wants to check how GDB
+    # displays the 'quit' message relative to the prompt, this is much
+    # easier to do if the prompt is still in expect's buffers.
+    #
+    # The other special thing we do here is avoid printing a 'PASS'
+    # result.  The reason for this is so that the GDB output in the
+    # log file will match what a user should see, this makes it much
+    # easier to debug issues.  Obviously we could print a 'PASS' here
+    # as the text printed by expect is not considered part of GDB's
+    # output, so the pattern matching will work just fine... but, the
+    # log file becomes much harder to read.
     send_gdb "\n"
     gdb_test_multiple "" "discard newline" {
 	-re "^\r\n" {
+	    exp_continue
+	}
+	-notransfer -re "^\[^\n\]*$::gdb_prompt $" {
 	}
     }