[gdb/testsuite] Require debug info for gdb.tui/tui-layout-asm-short-prog.exp

When running test-case gdb.tui/tui-layout-asm-short-prog.exp on SLE-12-SP3
aarch64, I run into:
...
FAIL: gdb.tui/tui-layout-asm-short-prog.exp: check asm box contents
FAIL: gdb.tui/tui-layout-asm-short-prog.exp: check asm box contents again
...
due to:
...
(gdb) file tui-layout-asm-short-prog^M
Reading symbols from tui-layout-asm-short-prog...^M
(No debugging symbols found in tui-layout-asm-short-prog)^M
...

I managed to reproduce the same behaviour on openSUSE Leap 15.4 x86_64, by
removing the debug option.

Fix this by making the test-case unsupported if no debug info is found.

Tested on x86_64-linux.

gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.exp

@@ -32,6 +32,11 @@ if {![Term::prepare_for_tui]} {
     return
 }
 
+if { $gdb_file_cmd_debug_info == "nodebug" } {
+    unsupported "no debug info"
+    return
+}
+
 # This puts us into TUI mode, and should display the ASM window.
 Term::command_no_prompt_prefix "layout asm"
 Term::check_box_contents "check asm box contents" 0 0 80 15 "<_start>"