[testrunner] FuchsiaTester to conditionally overwrite "command".

Only overwrite "command" if it doesn't exist, otherwise use the supplied "command" as is.

Bug: IN-1310

Change-Id: I662ab5678d865b614f1e628e0bd14c77100b291c
diff --git a/cmd/testrunner/tester.go b/cmd/testrunner/tester.go
index 150ee96..ec8748a 100644
--- a/cmd/testrunner/tester.go
+++ b/cmd/testrunner/tester.go
@@ -134,8 +134,10 @@
 }
 
 func (t *FuchsiaTester) Test(ctx context.Context, test testsharder.Test, stdout io.Writer, stderr io.Writer) error {
-	name := path.Base(test.Path)
-	test.Command = []string{"runtests", "-t", name, "-o", t.remoteOutputDir + "runtests"}
+	if len(test.Command) == 0 {
+		name := path.Base(test.Path)
+		test.Command = []string{"runtests", "-t", name, "-o", t.remoteOutputDir + "runtests"}
+	}
 	return t.delegate.Test(ctx, test, stdout, stderr)
 }