Work around concurrent map write issue in tests.

[sipsma]

The merge diff tests were using some common LLB state objects across
multiple test cases, which each have their own buildkit client and ran
in parallel. The LLB client library does not appear to currently work
with such use cases where LLB states are shared, with errors about
concurrent map writes to marshal state constraints occasionally being
hit during test runs.

Until that issue is addressed (or it's made explicit that the LLB client
library is not expected to work in such a way), this works around the
problem by not sharing states and instead using common funcs that return
distinct state instances.

Signed-off-by: Erik Sipsma erik@sipsma.dev

The issue this is working around is tracked here.

[crazy-max]

@sipsma Are you aware of this one https://github.com/moby/buildkit/runs/5194094096?check_suite_focus=true#step:6:1058?:

=== CONT  TestIntegration/TestDiffOfMerges/worker=containerd-snapshotter-stargz
    mergediff_test.go:1285: 
        	Error Trace:	client_test.go:4696
        	            				mergediff_test.go:1285
        	            				run.go:201
        	Error:      	Received unexpected error:
        	            	rpc error: code = Unavailable desc = error reading from server: EOF
        	            	failed to receive status
        	            	github.com/moby/buildkit/client.(*Client).solve.func4
        	            		/src/client/solve.go:261
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:57
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1581
        	Test:       	TestIntegration/TestDiffOfMerges/worker=containerd-snapshotter-starg

[sipsma]

@crazy-max No that looks to be an unrelated issue with the progress controller, thanks for letting me know. PR for a fix to it is here: #2641