DM: move skiatest functions into separate file.

Motivation: allows reuse.

Change-Id: I5732305a213acd83de4ba4e84e0ae27d094cf649
Reviewed-on: https://skia-review.googlesource.com/57241
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index b5ae1de..a65a211 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1332,6 +1332,7 @@
     test_app("dm") {
       sources = [
         "dm/DM.cpp",
+        "dm/DMGpuTestProcs.cpp",
         "dm/DMJsonWriter.cpp",
         "dm/DMSrcSink.cpp",
       ]