Enable creating image as part of the test

[tonistiigi]

Make sure tested codepath is longer and doesn't end without trying to export the build result.

Local run:

BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.12.0/run=1
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.12.0/run=1-8                 1    80665038297 ns/op    3005320 B/op       5638 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.12.0/run=2
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.12.0/run=2-8                 1    17462715767 ns/op    2784432 B/op       5266 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.13.0/run=1
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.13.0/run=1-8                 1    19406766775 ns/op    2808448 B/op       5564 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.13.0/run=2
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.13.0/run=2-8                 1    18879301191 ns/op    2817800 B/op       5598 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.14.0/run=1
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.14.0/run=1-8                 1    16501887018 ns/op    2286864 B/op       5570 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.14.0/run=2
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.14.0/run=2-8                 1    16100322279 ns/op    2283176 B/op       5561 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.15.2/run=1
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.15.2/run=1-8                 1    16594337035 ns/op    2285336 B/op       5559 allocs/op
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.15.2/run=2
BenchmarkBuild/BenchmarkBuildBreaker64/ref=v0.15.2/run=2-8                 1    16261203568 ns/op    2281744 B/op       5567 allocs/op
BenchmarkDaemon

[tonistiigi]

Additionally, I don't think the current setup is ideal for testing:

• We should use a local source build as it is much more common, unless testing remote case specifically
• We shouldn't use relatively big go build for this, unless we want to have a test case specific to go (eg. with cache mounts). Pulling down and extracting the Go image and compiling the source code skews the test results for such builds. If the task is to test high parallelization of buildkit, most of the resources should be used by buildkit itself and not external tasks.