bake: fix testing json formatted output

[tonistiigi]

Because the test checked for combinedoutput, it
could contain internal warning messages in stderr. JSON output is guaranteed in stdout.

Fixes #2575

This was the debug output:

=== FAIL: tests TestIntegration/TestBakeCallCheckFlag/worker=docker-container (3.96s)
    bake.go:1086: 
        	Error Trace:	/src/tests/bake.go:1086
        	            				/src/vendor/github.com/moby/buildkit/util/testutil/integration/run.go:96
        	            				/src/vendor/github.com/moby/buildkit/util/testutil/integration/run.go:211
        	Error:      	Received unexpected error:
        	            	invalid character '/' after top-level value
        	Test:       	TestIntegration/TestBakeCallCheckFlag/worker=docker-container
        	Messages:   	2024/07/09 00:57:04 http2: server connection error from localhost: connection error: PROTOCOL_ERROR
        	            	{
        	            	  "group": {
        	            	    "default": {
        	            	      "targets": [
        	            	        "build",
        	            	        "another"
        	            	      ]
        	            	    }