wasm: use emrun to run tests.html

emrun automates starting a web server, opening the .html
in the browser, capturing stdout and stderr, and the
return code from main().

https://emscripten.org/docs/compiling/Running-html-files-with-emrun.html

Change-Id: I03bb04e64f84e599fcfcbfbb4d8e9a4584e525f5
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/199555
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
diff --git a/build/emscripten b/build/emscripten
index cf0d923..be013a8 100644
--- a/build/emscripten
+++ b/build/emscripten
@@ -1,12 +1,11 @@
 cc      = emcc
 cxx     = em++
 cflags  = -g3 -s WASM=1
-ldflags = -g3 -s WASM=1 --embed-file profiles
+ldflags = -g3 -s WASM=1 --embed-file profiles --emrun
 exe     = .html
 out     = out/emscripten$mode
 
 rule run
-    command = touch $out
-    description = skipping $in
+    command = emrun $in > $out
 
 include build/common