Add regression test for vvp quiet flag.

ivtest/gold/vvp_quiet_mode-vvp-stderr.gold

@@ -0,0 +1,3 @@
+This should be suppressed
+This should be suppressed
+ivltests/vvp_quiet_mode.v:7: $finish(1) called at 0 (1s)

ivtest/gold/vvp_quiet_mode-vvp-stdout.gold

@@ -0,0 +1 @@
+This should be displayed

ivtest/ivltests/vvp_quiet_mode.v

@@ -0,0 +1,10 @@
+module test();
+
+initial begin
+  $display("This should be suppressed");
+  $fdisplay(32'h00000001, "This should be suppressed");
+  $fdisplay(32'h80000001, "This should be displayed");
+  $finish(1);
+end
+
+endmodule

ivtest/regress-vvp.list

@@ -276,6 +276,7 @@ vams_abs3			vvp_tests/vams_abs3.json
 vams_abs3-vlog95		vvp_tests/vams_abs3-vlog95.json
 vardly_undefined_vec		vvp_tests/vardly_undefined_vec.json
 va_math				vvp_tests/va_math.json
+vvp_quiet_mode			vvp_tests/vvp_quiet_mode.json
 warn_opt_sys_tf			vvp_tests/warn_opt_sys_tf.json
 wreal				vvp_tests/wreal.json
 writemem-invalid		vvp_tests/writemem-invalid.json

ivtest/vvp_tests/vvp_quiet_mode.json

@@ -0,0 +1,6 @@
+{
+    "type"          : "normal",
+    "source"        : "vvp_quiet_mode.v",
+    "gold"          : "vvp_quiet_mode",
+    "vvp-args" : [ "-ql-" ]
+}