TST Fix expected xsplot test data: add missing reactions
TST Fix expected xsplot test data: add missing reactions

Oxygen reactions 91 and 103 were missing from the test, but
reaction 3 was included in their place. Not sure how this didn't
fail earlier...

Also expanded the MTdescrip section for this isotope for clarity

CORE-GATECH-GROUP/serpent-tools,CORE-GATECH-GROUP/serpent-tools