man: support building manual page index cache

The apropos software is useful to get a list of manpages matching a
description or to get a list of all manpages. The latter feature is
used by Emacs to get manpage completion (`M-x man`).

To have apropos working, a database of all available manpages must be
built with mandb. This is what this commits does.

A similar change was done for NixOS:
edc6a76cc0

tests/modules/programs/man/apropos.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.man = {
+      enable = true;
+      generateCaches = true;
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.manpath
+
+      CACHE_DIR=$(cat $TESTED/home-files/.manpath | cut --delimiter=' ' --fields=3)
+
+      if [[ ! -f "$CACHE_DIR/index.bt" ]]; then
+          fail "Expected man cache files to exist (in $CACHE_DIR) but they were not found."
+      fi
+    '';
+  };
+}

tests/modules/programs/man/default.nix

@@ -0,0 +1,4 @@
+{
+  man-apropos = ./apropos.nix;
+  man-no-manpath = ./no-manpath.nix;
+}

tests/modules/programs/man/no-manpath.nix

@@ -0,0 +1,13 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.man = { enable = true; };
+
+    nmt.script = ''
+      assertPathNotExists home-files/.manpath
+    '';
+  };
+}