client: add test case for multiple parallel builds.
client: add test case for multiple parallel builds.

It appears that multiple parallel builds suffer some cross talk between their
mount/snapshots e.g. this new case currently fails because iterations see files
in `/src/` which correspond to other iterations (i.e. iteration 1 might see a
file called "test2.txt"). The `ls -i` and `stat` output also confirms that
where tests see the same filename they are also apparently seeing the same
inode.

Note that each solve uses a `SolverOpt.SharedKey` of the (unique) source
directory so there should be no clashes. Use of `llb.sharedKeyHint()` is
avoided since using a unique one of these hides the issue (AIUI uniqueness of
`sharedKeyHint` is only required with a session, not between them).

The shell fragment within the test is a bit crazy, it tries to output as much
info about what it saw vs what was expected to aid debugging.

I have also been seeing cases in my own code where `cacheContext.checksum`
fails the `not found` test at the top, which seems like either the wrong thing
is mounted or it has gone away or otherwise been mutated unexpectedly. I had
been hoping that this test case would also reproduce this, but so far it has
not.

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc