[gdb/testsuite] Fix spawn in tuiterm.exp

[gdb/testsuite] Fix spawn in tuiterm.exp

When running gdb.stabs/weird.exp by itself it passes, but if we run a gdb.tui
test before it, we get:
...
$ make check RUNTESTFLAGS="gdb.stabs/weird.exp gdb.tui/basic.exp"
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.tui/basic.exp ...
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.stabs/weird.exp ...
ERROR: Couldn't make test case. -1 {spawn failed}
...

In more detail, using -v:
...
Executing on build: sed  -f aout.sed weird.def weird.s (timeout = 300)
builtin_spawn [open ...]^M
pid is 19060 19061 -19060 -19061
spawn -open file10 failed, 1 can't read "spawn_out(slave,name)": \
  no such variable, can't read "spawn_out(slave,name)": no such variable
    while executing
"set gdb_spawn_name $spawn_out(slave,name)"
    (procedure "spawn" line 5)
    invoked from within
"spawn -ignore SIGHUP -leaveopen file10"
    invoked from within
"catch "spawn -ignore SIGHUP -leaveopen $id" result2"
ERROR: Couldn't make test case. -1 {spawn failed}
...

When running the gdb.tui test, spawn gets renamed to a local version from
lib/tuiterm.exp.  The local version calls expect's spawn, and then makes the
local spawn_out(slave,name) variable accessible in the global variable
gdb_spawn_name.

However, the weird.exp test-case uses remote_exec build, which ends up using
local_exec, which given that there's input/output redirection uses open:
...
    set result [catch {open "| ${commandline} $inp |& tee $outpf" RDONLY} id]
...
followed by spawn using the -leaveopen option:
...
    set result [catch "spawn -ignore SIGHUP -leaveopen $id" result2]
...
which apparently has the effect that spawn_out(slave,name) is not set.

Fix this in the lib/tuiterm.exp local spawn proc by detecting the case that
spawn_out(slave,name) is not set, and handling it accordingly.

Tested gdb.stabs/weird.exp and gdb.tui/*.exp on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-27  Tom de Vries  <tdevries@suse.de>

	* lib/tuiterm.exp (spawn): Handle case that spawn_out(slave,name) is
	not set.