[gdb/testsuite] Add test-case gdb.tui/color-prompt.exp

Add a test-case that sets a prompt with color in TUI.

The line containing the prompt is shown by get_line_with_attrs as follows:
...
<fg:31>(gdb) <fg:default>
...

The 31 means red, but only for foreground colors, for background colors 41
means red.

Make this more readable by using color names for both foreground and
background, such that we have instead:
....
<fg:red>(gdb) <fg:default>
...

Tested on x86_64-linux.

gdb/testsuite/gdb.tui/color-prompt.exp

@@ -0,0 +1,32 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check using a prompt with color in TUI.
+
+tuiterm_env
+
+Term::clean_restart 24 80
+
+# Set colored prompt.
+if {![Term::enter_tui]} {
+    unsupported "TUI not supported"
+    return
+}
+
+Term::command "set prompt \\033\[31m(gdb) \\033\[0m"
+
+set line [Term::get_line_with_attrs $Term::_cur_row]
+gdb_assert { [regexp "^<fg:red>$gdb_prompt <fg:default> *$" $line] } \
+    "prompt with color"

gdb/testsuite/gdb.tui/tuiterm.exp

@@ -699,8 +699,30 @@ proc test_attrs {} {
 	    5 1
 	    25 0
 	}
+	fg {
+	    30 black
+	    31 red
+	    32 green
+	    33 yellow
+	    34 blue
+	    35 magenta
+	    36 cyan
+	    37 white
+	    39 default
+	}
+	bg {
+	    40 black
+	    41 red
+	    42 green
+	    43 yellow
+	    44 blue
+	    45 magenta
+	    46 cyan
+	    47 white
+	    49 default
+	}
     } {
-	setup_terminal 8 1
+	setup_terminal 12 1
 	set re ""
 	foreach { arg val } $vals {
 	    Term::_insert "a"

gdb/testsuite/lib/tuiterm.exp

@@ -561,6 +561,37 @@ namespace eval Term {
 	}
     }
 
+    # Translate the color numbers as used in proc _csi_m to a name.
+    proc _color_attr { n } {
+	switch -exact -- $n {
+	    0 {
+		return black
+	    }
+	    1 {
+		return red
+	    }
+	    2 {
+		return green
+	    }
+	    3 {
+		return yellow
+	    }
+	    4 {
+		return blue
+	    }
+	    5 {
+		return magenta
+	    }
+	    6 {
+		return cyan
+	    }
+	    7 {
+		return white
+	    }
+	    default { error "unsupported color number: $n" }
+	}
+    }
+
     # Select Graphic Rendition.
     #
     # https://vt100.net/docs/vt510-rm/SGR.html
@@ -607,13 +638,13 @@ namespace eval Term {
 			set _attrs(invisible) 0
 		    }
 		    30 - 31 - 32 - 33 - 34 - 35 - 36 - 37 {
-			set _attrs(fg) $item
+			set _attrs(fg) [_color_attr [expr $item - 30]]
 		    }
 		    39 {
 			set _attrs(fg) default
 		    }
 		    40 - 41 - 42 - 43 - 44 - 45 - 46 - 47 {
-			set _attrs(bg) $item
+			set _attrs(bg) [_color_attr [expr $item - 40]]
 		    }
 		    49 {
 			set _attrs(bg) default