[gdb/testsuite] Check spawn result in gdb.tui/tuiterm-2.exp

[gdb/testsuite] Check spawn result in gdb.tui/tuiterm-2.exp

Simon reported [1] that test-case gdb.tui/tuiterm-2.exp fails with an ERROR
if tclsh is missing.

Fix this by checking that spawning the gdb.tcl script succeeds.

While we're at it, also add some missing cleanup related to the spawned process.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://sourceware.org/pipermail/gdb-patches/2025-September/220736.html