[test_server,common_test] Fix cross cover mechansim

Update the interface for cross cover analysis (collection of cover
data over multiple tests) so it can be used via common_test and
directly with test server.

The concept of 'application' in the cross cover interface is removed
and replaced with an arbitrary Tag=atom() which identifies a test run.
[test_server] Add test of code cover mechanism
[test_server] Update documentation about cross cover

Add info about cross statement in cover file:
{cross,[{Tag,Modules}]}.

Add parameter Tests = [{Tag,Dir}] to test_server_ctrl:cross_cover_analyse
[common_test] Add test case for cross cover mechanism
[common_test] Add documentation for cross cover analysis
Fix subjectPublicKeyInfo type comment
[test_server] Don't use print/3 in cross_cover_analyse

Cross cover analysis is executed outside the scope of
test_server_ctrl, i.e. test_server_ctrl is not necessarily
started. This may cause test_server_ctrl:print/3 to hang. Instead use
io:fwrite directly (as it was before).
asn1_SUITE: Only run testExtensionAdditionGroup/1 once
testSetOptional: Correct test case for decoding of corrupt data

Eliminate the spawning of the process since the original bug is
fixed.
Simplify testSetOptional by introducing a roundtrip/2 function
Simplify testChoExtension by introducing a roundtrip/2 function
Simplify testSeqExtension.erl by introducing a roundtrip/2 function

Make the test suite more readable by introducing and using a
roundtrip/2 function.
Simplify testChoRecursive by introducing a roundtrip/2 function
Simplify testChoExternal by introducing a roundtrip/2 function
Simplify testEnumExt by introducing a roundtrip/2 function
Simplify testCompactBitString by introducing roundtrip functions

Also run the test case for all back-ends.
Simplify testConstraints by introducing helper functions
asn1 doc: Remove a reference to a section that has been removed
asn1rt_uper_bin: Correct incorrect skipping of extensions

Commit 1b622484ea984f3bc424d2a6760e2d961bfcf816 changed the type of
the extension bitmap for uper (from a tuple to a bitstring), but did
not do the the corresponding change to the run-time module.
Fix a bug in skipping of extensions

Extensions following a double bracket group were not skipped correctly.
Fix this bug and add tests to ensure that extensions are skipped
correctly.