Tests: Some coverage improvements

test_regress/t/t_preproc_debugi.v

@@ -8,3 +8,23 @@
 `define BAR(aa,bb) aa bb
 `FOO
 `BAR(aa,bb)
+
+`ifdef FOO
+`else
+`endif
+`ifndef FOO
+`elsif FOO
+`endif
+
+`define STRINGIFY(x) `"x`"
+`define CONCAT(a, b) a``b
+`STRINGIFY(x)
+`CONCAT(x,y)
+
+`undef FOO
+
+`undefineall
+
+`ifdef NEVER
+`error "should not get"
+`endif

test_regress/t/t_preproc_inc_fn_bad.out

@@ -0,0 +1,4 @@
+%Error: t/t_preproc_inc_fn_bad.v:7:10: Expecting include filename. Found: ELSE
+    7 | `include `else
+      |          ^~~~~
+%Error: Exiting due to

test_regress/t/t_preproc_inc_fn_bad.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(vlt => 1);
+
+lint(
+    fails => 1,
+    # The .vh file has the error, not the .v file
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_preproc_inc_fn_bad.v

@@ -0,0 +1,10 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2010 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+`include `else
+
+module t;
+endmodule