[FIX, INTERNAL]

on Windows the tests below might fail when OpenMS is configured with a
OPENMS_GET_TEST_DATA_PATH which contains a small device letter, e.g.

//#define OPENMS_GET_TEST_DATA_PATH(filename) (std::string("c:/dev/oms/source/TEST/data/") + filename).c_str()

since getLoadedFilePath() used to call Qt to create absolute paths and
this changes "c" to "C", making the string comparison like

TEST_EQUAL(di2.getLoadedFilePath(), OPENMS_GET_TEST_DATA_PATH("File_test_empty.txt"))

fail. The fix here is to modify/fix the path only if its not absolute,
thus for our tests it will stay untouched since OPENMS_GET_TEST_DATA_PATH
is always absolute.

Alternatively, one could modify each(!) test (e.g. convert to lower case
before comparing),
but this creates invalid paths on unix and is error prone (since the next
developer will surely forget to use it).

Failing tests are

123 - ConsensusXMLFile_test (Failed)
126 - DTA2DFile_test (Failed)
130 - FeatureXMLFile_test (Failed)
142 - MS2File_test (Failed)
143 - MSPFile_test (Failed)
149 - MzDataFile_test (Failed)
153 - MzMLFile_test (Failed)
158 - MzXMLFile_test (Failed)
Merge pull request #657 from cbielow/B_loadedFilePath

[FIX, INTERNAL] make "void DocumentIdentifier::setLoadedFilePath(const String & file_name)" not modify file_name if already absolute