[gdb/testsuite] Disable CLI styling by default in Term::prepare_tui

On x86_64-freebsd, with test-case gdb.tui/main.exp, I ran into a failure to
match the output of the file command, for which I submitted a patch [1].

However, after switching to TERM=ansiw for bsd, I could no longer reproduce
the problem.

Investigation showed that the difference was caused by CLI styling.

A command:
...
(gdb) file <filename>
...
gives an output:
...
Reading symbols from <filename>...
(gdb)
...

On x86_64-linux, with CLI styling I get:
...
Reading symbols from ^[[32m/data/vries/gdb/leap-15-6/build/gdb/testsuite/outputs/gdb.tui/main/main^[[39m^[[0;10m...
...

After disabling CLI styling using "set style enabled off", I simply get:
...
Reading symbols from /data/vries/gdb/leap-15-6/build/gdb/testsuite/outputs/gdb.tui/main/main...
...
and run into the same failure as on x86_64-freebsd.

The extra csi sequence "^[[32m" causes an additional matching attempt in
Term::wait_for, and the way we're currently matching relies on this:
...
send_gdb "file [standard_output_file $testfile]\n"
gdb_assert { [Term::wait_for "Reading symbols from"] } "file command"
...

Make the TUI testsuite more stable and matching more simple by disabling CLI
styling by default, and:
- fix the resulting fallout in test-cases gdb.tui/main.exp and
  gdb.tui/new-layout.exp, and
- re-enable CLI styling in the one test-case that needs it:
  gdb.tui/tui-disasm-styling.exp.

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2025-June/218942.html

gdb/testsuite/gdb.tui/main.exp

@@ -43,7 +43,10 @@ if {![Term::enter_tui]} {
 }
 
 send_gdb "file [standard_output_file $testfile]\n"
-gdb_assert { [Term::wait_for "Reading symbols from"] } "file command"
+# Matching the output is difficult because it may or may not wrap.  Simply
+# match the resulting prompt.
+gdb_assert { [Term::wait_for ""] } "file command"
+
 Term::check_contents "show main after file" \
     [string_to_regexp "|___[format %06d $nr]_$line"]
 

gdb/testsuite/gdb.tui/new-layout.exp

@@ -150,4 +150,7 @@ Term::check_box "before cmd_only: src box in src layout" 0 0 80 15
 Term::command "layout cmd_only"
 
 Term::command "layout src"
+
+# Flush out and check the resulting src box.
+Term::command "print 1"
 Term::check_box "after cmd_only: src box in src layout" 0 0 80 15

gdb/testsuite/gdb.tui/tui-disasm-styling.exp

@@ -43,6 +43,9 @@ if {![Term::enter_tui]} {
     return
 }
 
+# Proc enter_tui switches off styling, re-enable it.
+Term::command "set style enabled on"
+
 Term::command "layout asm"
 Term::check_box "asm box" 0 0 80 15
 

gdb/testsuite/lib/tuiterm.exp

@@ -1013,6 +1013,10 @@ namespace eval Term {
 
 	gdb_test_no_output "set tui border-kind ascii"
 	gdb_test_no_output "maint set tui-resize-message on"
+	# When matching GDB output using Term::wait_for, the number of
+	# matching attempts in wait_for can be influenced by CLI styling.
+	# Disable it by default to avoid this.
+	gdb_test_no_output "set style enabled off"
 	return 1
     }