Test sk_linear_to_srgb() less exhaustively in Debug mode.

It's pretty slow when not optimized... it can take ~5 minutes to run.
This makes it take ~2 seconds on my laptop.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2221803002

Review-Url: https://codereview.chromium.org/2221803002
diff --git a/tests/SRGBTest.cpp b/tests/SRGBTest.cpp
index 43ec027..77c1fb7 100644
--- a/tests/SRGBTest.cpp
+++ b/tests/SRGBTest.cpp
@@ -33,6 +33,7 @@
 
         union { float flt; uint32_t bits; } pun = { f };
         pun.bits++;
+        SkDEBUGCODE(pun.bits += 127);
         f = pun.flt;
     }
 }