fpspreadsheet: Extend formula reading test case to biff5 and biff2 and generalize it such that the test results are not affected by locale settings and rounding errors.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3096 8e941d3f-bd1b-0410-a28a-d453659cc2b4