Fix silent gdb.base/annota1.exp test coverage regression

Fix silent gdb.base/annota1.exp test coverage regression

This commit fixes a test coverage regression caused by:

 commit b001de2320446ec803b4ee5e0b9710b025b84469
 Author:     Andrew Burgess <andrew.burgess@embecosm.com>
 AuthorDate: Mon Nov 26 17:56:39 2018 +0000
 Commit:     Andrew Burgess <andrew.burgess@embecosm.com>
 CommitDate: Wed Dec 12 17:33:52 2018 +0000

     gdb: Update test pattern to deal with native-extended-gdbserver

While looking at a regression caused by a local patch I was working
on, I noticed this:

 \1a\1apre-prompt
 (gdb)
 \1a\1aprompt
 PASS: gdb.base/annota1.exp: breakpoint info
 PASS: gdb.base/annota1.exp: run until main breakpoint
 run

 \1a\1apost-prompt
 Starting program: /home/pedro/gdb/mygit/build/gdb/testsuite/outputs/gdb.base/annota1/annota1
 next

Note how above, we get the "run until main breakpoint" pass even
before "run" shows up in the log!  The issue is that the test isn't
really testing anything, it always passes regardless of the gdb
output.

There are a few problems here, fixed by this commit:

 - using {} to build the list for [join], when the strings we're
   joining include variable names that must be expanded.  Such list
   need to be built with [list] instead.

 - [join] joins strings with a space character by default.  We need to
   pass the empty string as second parameter so that it just concats
   the strings.

 - doing too much in a "-re" (calling procedures), which doesn't work
   correctly.  I've seen this before and never digged deeper into why.
   Probably something to do with how gdb_test_multiple is implemented.
   Regardless, easy and clear enough to build the pattern first into a
   variable.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* gdb.base/annota1.exp: Build list using [list] instead of {}.
	Tell [join] to join with no character.  Build expected pattern in
	separate variable instead of in the -re expression directly.

Change-Id: Ib3c89290f0e9ae4a0a43422853fcd4a7a7e12b18