tui: Add st to the always 256-colour capable list.

Also comment and augment some terminal colour tests.

src/nvim/tui/tui.c

@@ -2459,7 +2459,7 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
   if (unibi_get_num(ut, unibi_max_colors) < 256) {
     // See http://fedoraproject.org/wiki/Features/256_Color_Terminals for
     // more on this.
-    if (konsole || mate || xterm || gnome || rxvt
+    if (konsole || mate || xterm || gnome || rxvt || st
         || linuxvt  // Linux 4.8+ supports 256-colour SGR.
         || (colorterm && strstr(colorterm, "256"))
         || (term && strstr(term, "256"))

test/functional/terminal/tui_spec.lua

@@ -399,7 +399,15 @@ describe("tui 't_Co' (terminal colors)", function()
     assert_term_colors("linux-16color", nil, 256)
   end)
 
+  it("TERM=linux-256color uses 256 colors", function()
+    assert_term_colors("linux-256color", nil, 256)
+  end)
+
   -- screen and tmux:
+  --
+  -- FreeBSD falls back to the built-in screen-256colour entry.
+  -- Linux and MacOS have a screen entry in external terminfo with 8 colours,
+  -- which is raised to 16 by COLORTERM.
 
   it("TERM=screen no COLORTERM uses 8/256 colors", function()
     if is_freebsd then
@@ -460,35 +468,48 @@ describe("tui 't_Co' (terminal colors)", function()
   end)
 
   -- rxvt and stterm:
+  --
+  -- FreeBSD and MacOS fall back to the built-in rxvt-256color and
+  -- st-256colour entries.
+  -- Linux has an rxvt, an st, and an st-16color entry in external terminfo
+  -- with 8, 8, and 16 colours respectively, which are raised to 256.
 
   it("TERM=rxvt no COLORTERM uses 256 colors", function()
     assert_term_colors("rxvt", nil, 256)
   end)
 
+  it("TERM=rxvt COLORTERM=rxvt uses 256 colors", function()
+    assert_term_colors("rxvt", "rxvt", 256)
+  end)
+
   it("TERM=rxvt-256color uses 256 colors", function()
     assert_term_colors("rxvt-256color", nil, 256)
   end)
 
-  it("TERM=st no COLORTERM uses 8/256 colors", function()
+  it("TERM=st no COLORTERM uses 256 colors", function()
-    if is_freebsd then
     assert_term_colors("st", nil, 256)
-    else
-      assert_term_colors("st", nil, 8)
-    end
   end)
 
-  it("TERM=st COLORTERM=st uses 16/256 colors", function()
+  it("TERM=st COLORTERM=st uses 256 colors", function()
-    if is_freebsd then
     assert_term_colors("st", "st", 256)
-    else
-      assert_term_colors("st", "st", 16)
-    end
   end)
 
   it("TERM=st COLORTERM=st-256color uses 256 colors", function()
     assert_term_colors("st", "st-256color", 256)
   end)
 
+  it("TERM=st-16color no COLORTERM uses 8/256 colors", function()
+    assert_term_colors("st", nil, 256)
+  end)
+
+  it("TERM=st-16color COLORTERM=st uses 16/256 colors", function()
+    assert_term_colors("st", "st", 256)
+  end)
+
+  it("TERM=st-16color COLORTERM=st-256color uses 256 colors", function()
+    assert_term_colors("st", "st-256color", 256)
+  end)
+
   it("TERM=st-256color uses 256 colors", function()
     assert_term_colors("st-256color", nil, 256)
   end)
@@ -499,6 +520,10 @@ describe("tui 't_Co' (terminal colors)", function()
     assert_term_colors("interix", nil, 8)
   end)
 
+  it("TERM=iTerm.app uses 256 colors", function()
+    assert_term_colors("iTerm.app", nil, 256)
+  end)
+
   it("TERM=iterm uses 256 colors", function()
     assert_term_colors("iterm", nil, 256)
   end)