Run headers tests only on demand

[mloskot]

Disable headers tests on CI services and during regression tests.
Refines #525

[awulkiew]

What do you think about simply removing this script and relying on manually running tests in self_contained_headers directory? This way one wouldn't have to create any environment variable so the command would be shorter.

[mloskot]

I planned to add a CI build job on Travis CI or AppVeyor Linux where I would define TEST_HEADERS.
I can equally run cd self_contained_headers && b2.

So, no objection to remove it.

I will update this PR.

[mloskot]

@awulkiew I see you have added 1.69 milestone. Since you aim to merge this into master too, I will leave merge of this to you.

[awulkiew]

Thanks.