Added tests for generating valid and unique xls sheet names
Added tests for generating valid and unique xls sheet names

ultimateprogramer/formhub,eHealthAfrica/onadata,awemulya/fieldsight-kobocat,spatialdev/onadata,makinacorpus/formhub,GeoODK/onadata,eHealthAfrica/onadata,hnjamba/onaclone,GeoODK/onadata,ehealthafrica-ci/formhub,ultimateprogramer/formhub,jomolinare/kobocat,hnjamba/onaclone,hnjamba/onaclone,spatialdev/onadata,spatialdev/onadata,piqoni/onadata,makinacorpus/formhub,kobotoolbox/kobocat,SEL-Columbia/formhub,eHealthAfrica/onadata,jomolinare/kobocat,qlands/onadata,GeoODK/formhub,mainakibui/kobocat,smn/onadata,wesley1001/formhub,spatialdev/onadata,ehealthafrica-ci/formhub,ehealthafrica-ci/formhub,awemulya/fieldsight-kobocat,makinacorpus/formhub,mainakibui/kobocat,wesley1001/formhub,ehealthafrica-ci/formhub,eHealthAfrica/formhub,eHealthAfrica/formhub,SEL-Columbia/formhub,kobotoolbox/kobocat,kobotoolbox/kobocat,ehealthafrica-ci/onadata,SEL-Columbia/formhub,sounay/flaminggo-test,awemulya/fieldsight-kobocat,jomolinare/kobocat,piqoni/onadata,wesley1001/formhub,qlands/onadata,GeoODK/formhub,qlands/onadata,GeoODK/onadata,GeoODK/formhub,ehealthafrica-ci/onadata,makinacorpus/formhub,kobotoolbox/kobocat,piqoni/onadata,GeoODK/onadata,qlands/onadata,jomolinare/kobocat,ultimateprogramer/formhub,smn/onadata,smn/onadata,ehealthafrica-ci/onadata,awemulya/fieldsight-kobocat,piqoni/onadata,sounay/flaminggo-test,smn/onadata,sounay/flaminggo-test,mainakibui/kobocat,eHealthAfrica/formhub,ultimateprogramer/formhub,SEL-Columbia/formhub,hnjamba/onaclone,sounay/flaminggo-test,ehealthafrica-ci/onadata,mainakibui/kobocat,wesley1001/formhub,eHealthAfrica/formhub