Add reproducer for VUnit#695

Reproduces error on modelsim, while pass on ghdl.

vunit/vhdl/verification_components/test/avalon_tb.vhd

@@ -0,0 +1,75 @@
+-- This Source Code Form is subject to the terms of the Mozilla Public
+-- License, v. 2.0. If a copy of the MPL was not distributed with this file,
+-- You can obtain one at http://mozilla.org/MPL/2.0/.
+--
+-- Copyright (c) 2014-2020, Lars Asplund [email protected]
+
+-- This test bench is to reproduce issue with pop form empty queue in modelsim.
+library ieee;
+use ieee.std_logic_1164.all;
+
+library vunit_lib;
+context vunit_lib.vunit_context;
+context vunit_lib.vc_context;
+
+entity avalon_tb is
+    generic (runner_cfg : string);
+end avalon_tb;
+
+architecture testbench of avalon_tb is
+
+    -- Avalon-MM Slave --
+    signal av_address : std_logic_vector(31 downto 0);
+    signal av_write : std_logic := '0';
+    signal av_writedata : std_logic_vector(31 downto 0) := (others=> '0');
+    signal av_read : std_logic := '0';
+    signal av_readdata : std_logic_vector(31 downto 0) := (others=> '0');
+    signal av_byteenable : std_logic_vector(3 downto 0);
+    signal av_burstcount : std_logic_vector(3 downto 0);
+
+    constant avalon_bus : bus_master_t := new_bus(data_length => 32, address_length => av_address'length);
+
+    signal clk : std_logic := '0';
+
+    constant CLK_period : time := 20 ns;
+
+begin
+    avalon_master : entity vunit_lib.avalon_master
+    generic map (
+        bus_handle => avalon_bus,
+        use_readdatavalid => false,
+        fixed_read_latency => 0
+    )
+    port map (
+      clk => clk,
+      address => av_address,
+      byteenable => av_byteenable,
+      burstcount => av_burstcount,
+      write => av_write,
+      writedata => av_writedata,
+      read => av_read,
+      readdata => av_readdata,
+      readdatavalid => '0',
+      waitrequest => '0'
+    );
+
+    -- Clock process definitions
+    CLK_process: process
+    begin
+        clk <= '0';
+        wait for CLK_period/2;
+        clk <= '1';
+        wait for CLK_period/2;
+    end process;
+
+    tests: process
+    begin
+        test_runner_setup(runner, runner_cfg);
+
+        wait for CLK_period*2;
+
+        check_bus(net, avalon_bus, 0, (0 to 31 => '0'));
+
+        test_runner_cleanup(runner);
+    end process;
+end;