Use software renderer in embedder unittests (#6928)
Use software renderer in embedder unittests (#6928)

Reduces spurious error log messages in GLContextMakeCurrent() attempting
set up the GR context:

  [ERROR:flutter/shell/gpu/gpu_surface_gl.cc(42)] Could not make the context current to setup the gr context.
flutter/engine,jamesr/sky_engine,Hixie/sky_engine,cdotstout/sky_engine,mikejurka/engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,flutter/engine,chinmaygarde/sky_engine,cdotstout/sky_engine,devoncarew/engine,flutter/engine,jamesr/flutter_engine,cdotstout/sky_engine,aam/engine,jamesr/sky_engine,cdotstout/sky_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,tvolkert/engine,jamesr/flutter_engine,tvolkert/engine,devoncarew/engine,rmacnak-google/engine,tvolkert/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,jamesr/sky_engine,jamesr/flutter_engine,aam/engine,chinmaygarde/flutter_engine,jamesr/flutter_engine,tvolkert/engine,flutter/engine,devoncarew/sky_engine,jamesr/flutter_engine,Hixie/sky_engine,rmacnak-google/engine,jamesr/flutter_engine,mikejurka/engine,flutter/engine,jamesr/sky_engine,mikejurka/engine,chinmaygarde/flutter_engine,rmacnak-google/engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,mikejurka/engine,jason-simmons/flutter_engine,aam/engine,devoncarew/sky_engine,jamesr/flutter_engine,aam/engine,chinmaygarde/sky_engine,devoncarew/sky_engine,mikejurka/engine,mikejurka/engine,jason-simmons/flutter_engine,jason-simmons/sky_engine,flutter/engine,Hixie/sky_engine,Hixie/sky_engine,aam/engine,jason-simmons/flutter_engine,aam/engine,Hixie/sky_engine,jamesr/flutter_engine,chinmaygarde/flutter_engine,aam/engine,flutter/engine,devoncarew/engine,mikejurka/engine,cdotstout/sky_engine,devoncarew/sky_engine,jason-simmons/sky_engine,cdotstout/sky_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,tvolkert/engine,devoncarew/engine,chinmaygarde/sky_engine,tvolkert/engine,cdotstout/sky_engine,devoncarew/engine,mikejurka/engine,mikejurka/engine,tvolkert/engine,jamesr/sky_engine,jason-simmons/flutter_engine,devoncarew/engine,jamesr/flutter_engine,aam/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,devoncarew/engine,chinmaygarde/flutter_engine,Hixie/sky_engine,rmacnak-google/engine,rmacnak-google/engine,Hixie/sky_engine,devoncarew/sky_engine,jason-simmons/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,jason-simmons/sky_engine,rmacnak-google/engine,chinmaygarde/sky_engine,flutter/engine,rmacnak-google/engine,devoncarew/sky_engine,jason-simmons/flutter_engine