MDL-39020 phpunit: Rename incorrect filename

Rename externallib_tests.php to externallib_test.php
Fix failing unit tests
MDL-40386: Fix for updating timemodified field in book_chapters
MDL-40434 calculatedmulti multiple-response questions not randomised
MDL-40472: Clean all codechecker warnings on mod_assign
MDL-38314 repository: Unit Tests for delete_all_for_context()
MDL-38314 repository: Delete repository instances on context deletion
MDL-38314 repository: Remove orphan repository instances
MDL-38314 repository: Improving performance of upgrade script
MDL-36479 quiz regrade: use progress bar to prevent time-outs
MDL-40318 quiz intro comprising just an impage was not shown

Also, ensure that we don't output an empty box if there are no access
messages.
MDL-40443 setType notice in quiz reports.
MDL-40392 Navigation -> my courses listing tests

This introduces some behat tests to verify that
the display of enrolled courses in the navigation
block works as expected, both when a plain list
or a nested list is configured.

Personally I'm not happy with the way to select
expected text via css element. I'd love to see
something in an higher level like

I should see the text|link "xxxxx" in the "yyyy" "element"

with element being "block, header, footer, main...."

But that's another issue.
MDL-40499 Edit question from 1.9 backup breaks if images in subdirs
MDL-40512 SCORM: prevent undefined warning
MDL-40527 enrol meta: respect $CFG->courselistshortnames

Thanks to James Henestofel for working this out in MDL-37558. That issue
got complex, so I am just submitting this simple part of it for
integration.
MDL-40488 improve $CFG->admin docs
Moodle release 2.5.1
MDL-39832 Files: Revert MDL-39688 commit, 161be9206571787557eb5c11ee440c9e6ae93ca8

MDL-39688 has been successful in providing a solution for the issue but further
investigations done in MDL-39832 have got the evidence that the given solution
is more likely a workaround.
MDL-39832 Files: Fixed ETag format according with RFC2616.

ETag must be double quoted,
http://tools.ietf.org/html/rfc2616#section-3.11:

      entity-tag = [ weak ] opaque-tag
      weak       = "W/"
      opaque-tag = quoted-string
MDL-40450 update links to TinyMCE 3.x docs