sim: testsuite: skip tests when the port is disabled

If the port hasn't been enabled, don't try to run its tests.  Making
this dynamic simplifies the test harnesses and avoids duplicating a
bunch of target tuple checks.

sim/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-04-08  Mike Frysinger  <vapier@gentoo.org>
+
+	* lib/sim-defs.exp (run_sim_test): Return if sim_tool_path does not
+	exist.
+
 2021-04-08  Mike Frysinger  <vapier@gentoo.org>
 
 	* lib/sim-defs.exp (sim_tool_path): New function.

sim/testsuite/lib/sim-defs.exp

@@ -205,6 +205,11 @@ proc run_sim_test { name requested_machs } {
     global global_cc_options
     global global_sim_options
 
+    if ![file exists [sim_tool_path]] {
+	unsupported "$name: missing simulator [sim_tool_path]"
+	return
+    }
+
     if [string match "*/*" $name] {
 	set file $name
 	set name [file tail $name]