For some testing of the sandbox, we might need to expect an existing directory. One example is the SNAP env variable https://searchfox.org/mozilla-central/rev/3c8a7970944daaf917b547dffc0790bcd37cadc1/security/sandbox/test/browser_snap.ini#8 and its handling in https://searchfox.org/mozilla-central/rev/3c8a7970944daaf917b547dffc0790bcd37cadc1/security/sandbox/test/browser_content_sandbox_fs_tests.js#644-661

A side effect of having to deal with that within the test is:

• it's more code to just deal within the test
• this is code that is run after the sandbox is created, so we have to circumvent some of the sandboxing in place
• we have to be a bit more lax in the sandbox https://searchfox.org/mozilla-central/diff/bf8b8ae67952fbe0cdf430679e1004fc0bc4a985/security/sandbox/linux/broker/SandboxBrokerPolicyFactory.cpp#569-578 with AddFutureDir while it should actually not be needed

Depends on D137455

Backed out for causing mochitest failures.

Backout link: https://hg.mozilla.org/integration/autoland/rev/08899f653c5a8a557a2c0bf00f1526e4cb66a333

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Crunnable&revision=4caa5563e76d57ce249acdf526ee1ea5c4ee57e0&selectedTaskRun=ZifDr9rDQjuv9Gu0lBcugw.0

Link to failure log:
https://treeherder.mozilla.org/logviewer?job_id=367071909&repo=autoland&lineNumber=2341
https://treeherder.mozilla.org/logviewer?job_id=367071021&repo=autoland&lineNumber=2349

https://hg.mozilla.org/mozilla-central/rev/285dcf4c6ecf