[gdb/testsuite] Add gdb.dwarf2/dwz-unused-pu.exp

Add a new test-case gdb.dwarf2/dwz-unused-pu.exp that checks that a symbol
from an unused PU is not accessible.

Passes with the relevant target boards:
- unix (using the cooked index),
- readnow (using no index at all),
- cc-with-gdb-index (using .gdb_index), and
- cc-with-debug-names (using .debug_names).

Tested on x86_64-linux.

gdb/testsuite/gdb.dwarf2/dwz-unused-pu.exp

@@ -0,0 +1,75 @@
+# Copyright 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that a symbol from an unused PU is not accessible.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile main.c -dw.S
+
+# Create the DWARF.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels partial_label int_label int_label2
+
+    cu {} {
+	compile_unit {{language @DW_LANG_C}} {
+	    subprogram {
+		{MACRO_AT_func { main }}
+	    }
+	}
+    }
+
+    cu {} {
+	partial_unit {} {
+	    int_label: base_type {
+		{name int}
+		{byte_size 4 sdata}
+		{encoding @DW_ATE_signed}
+	    }
+
+	    constant {
+		{name the_int}
+		{type :$int_label}
+		{const_value 99 data1}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile \
+	  [list $asm_file $srcfile] {nodebug}] } {
+    return -1
+}
+
+set cmd "p the_int"
+set re \
+    [string_to_regexp \
+	 {No symbol "the_int" in current context.}]
+
+# Check that the unreferenced PU is not expanded.
+gdb_test $cmd $re
+
+# This should expand all CUs and referenced PUs, but not an unreferenced PU.
+gdb_test_no_output "maint expand-symtabs"
+
+# Flush the symbol cache to do a new symbol lookup.
+gdb_test_no_output "maint flush symbol-cache"
+
+# Check that the unreferenced PU is not expanded after "maint expand-symtabs".
+gdb_test $cmd $re "$cmd, again"