[tests] create make target for coverage report

* ignore and delete (on make clean) all coverage related files
* do not make libs, cause they are directly compiled into the test binaries for coverage report
[os_xml] harmonize error messages
[os_xml] refactor name of variable attribute
[os_xml] limit output in overflow error message
[os_xml] add testcases
[os_xml] refactor max size of variables
[os_xml] refactor type of xml element as enum
[os_xml] fail on too long variable names and on null-content right away
[os_xml] fill error line in ApplyVariables()
[os_xml] do not copy strings in ApplyVariables
[os_xml] do not traverse the xml tree twice in ApplyVariables(); so variables must be defined before their usage
[os_xml] ensure behavior for by children divided multiple contents
[tests] replace tabs with spaces in test_os_xml.c
fix warnings about const discarding caused by os_xml api change

char *OS_GetOneContentforElement(OS_XML *_lxml, char **element_name)
->
char *OS_GetOneContentforElement(OS_XML *_lxml, const char **element_name)
[tests] fix not finding zlib.h in src/headers
[windows] fix os_xml related warnings
Merge pull request #173 from cgzones/os_xml2

os_xml refresh2