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

[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.