[gdb/testsuite] Add missing terminator in Dwarf::_macro_unit

When printing complaints with one of the execs from test-case
gdb.dwarf2/macro-source-path.exp, we run into:
...
$ gdb -q -batch \
    -iex "set complaints 100" \
    macro-source-path-clang14-dw4-absolute-cwd-32 \
    -ex "p main"
During symbol reading: debug info runs off end of .debug_macro section \
  [in module macro-source-path-clang14-dw4-absolute-cwd-32]
$1 = {int ()} 0x4004b7 <main>
...
and readelf complains more specifically:
...
Contents of the .debug_macro section:

  Offset:                      0
  Version:                     5
  Offset size:                 4
  Offset into .debug_line:     0xe3

 DW_MACRO_define - lineno : 0 macro : ONE 1
 DW_MACRO_define_strp - lineno : 0 macro : THREE 3
 DW_MACRO_start_file - lineno: 0 filenum: 1 filename: test.c
 DW_MACRO_define - lineno : 1 macro : TWO 2
 DW_MACRO_end_file
readelf: Error: .debug_macro section not zero terminated
...

Fix this by adding the missing terminator in Dwarf::_macro_unit.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.dwarf2/macro-source-path.exp

@@ -139,6 +139,17 @@ proc do_test { test_name lines_version DW_AT_name main_file_idx directories
 		return
 	    }
 
+	    with_complaints 5 {
+		gdb_test_multiple "print main" "no complaints" {
+		    -wrap -re "During symbol reading: .*" {
+			fail $gdb_test_name
+		    }
+		    -wrap -re "" {
+			pass $gdb_test_name
+		    }
+		}
+	    }
+
 	    if ![runto_main] {
 		return
 	    }

gdb/testsuite/lib/dwarf.exp

@@ -2252,6 +2252,8 @@ namespace eval Dwarf {
 	with_override Dwarf::end_file Dwarf::_macro_unit_end_file {
 	    uplevel $body
 	}}}}
+
+	_op .byte 0x0 "# End macro unit"
     }
 
     # Emit a DW_MACRO_define entry.