Move type converter unittests to separate binary

There's no reason to lump the converter unit tests into
mojo_common_unittests as the converters are not under mojo/common. It is
also inconvenient to have the test targets in mojo/common as consumers
like flutter that want to use mojo/common do not necessarily want to
depend on converters they don't use. This moves the converter test
targets into their own binary, mojo_converters_unittests, and adds that
to the unittest list so mojob.py runs it.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1436103002 .
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index eecb107..23682f4 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -63,6 +63,7 @@
   deps = [
     ":public_tests",
     "//mojo/common:mojo_common_unittests",
+    "//mojo/converters:mojo_converters_unittests",
     "//mojo/dart/embedder/test:dart_unittests",
     "//mojo/edk:tests",
     "//mojo/gles2:mgl_unittests",