test/dshell: Expose 'stderr' for run/tryRun

The compiler actually outputs on 'stderr' messages coming from 'pragma(msg)' so we should expose it.

test/dshell/test_shared.d

@@ -9,7 +9,7 @@ int main()
     }
 
     run("$DMD -m$MODEL -of$OUTPUT_BASE/a$EXE -defaultlib=libphobos2.so $EXTRA_FILES/test_shared.d");
-    run("$OUTPUT_BASE/a$EXE", stdout, [
+    run("$OUTPUT_BASE/a$EXE", stdout, stderr, [
         "LD_LIBRARY_PATH" : "../../phobos/generated/"~OS~'/'~BUILD~'/'~MODEL
     ]);
 

test/tools/dshell_prebuilt/dshell_prebuilt.d

@@ -151,7 +151,8 @@ void mkdirFor(string filename)
 Run the given command. The `tryRun` variants return the exit code, whereas the `run` variants
 will assert on a non-zero exit code.
 */
-auto tryRun(scope const(char[])[] args, File stdout = std.stdio.stdout, string[string] env = null)
+auto tryRun(scope const(char[])[] args, File stdout = std.stdio.stdout,
+            File stderr = std.stdio.stderr, string[string] env = null)
 {
     std.stdio.stdout.write("[RUN]");
     if (env)
@@ -168,26 +169,28 @@ auto tryRun(scope const(char[])[] args, File stdout = std.stdio.stdout, string[s
     }
     std.stdio.stdout.writeln();
     std.stdio.stdout.flush();
-    auto proc = spawnProcess(args, stdin, stdout, std.stdio.stderr, env);
+    auto proc = spawnProcess(args, stdin, stdout, stderr, env);
     return wait(proc);
 }
 /// ditto
-void run(scope const(char[])[] args, File stdout = std.stdio.stdout, string[string] env = null)
+void run(scope const(char[])[] args, File stdout = std.stdio.stdout,
+         File stderr = std.stdio.stderr, string[string] env = null)
 {
-    const exitCode = tryRun(args, stdout, env);
+    const exitCode = tryRun(args, stdout, stderr, env);
     if (exitCode != 0)
     {
         writefln("Error: last command exited with code %s", exitCode);
         assert(0, "last command failed");
     }
 }
 /// ditto
-void run(string cmd, File stdout = std.stdio.stdout, string[string] env = null)
+void run(string cmd, File stdout = std.stdio.stdout,
+         File stderr = std.stdio.stderr, string[string] env = null)
 {
     // TODO: option to disable this?
     if (SEP != "/")
         cmd = cmd.replace("/", SEP);
-    run(parseCommand(cmd), stdout, env);
+    run(parseCommand(cmd), stdout, stderr, env);
 }
 
 /**