MDL-40666 cohort: fix incorrect use of PARAM_NUMBER

In update_cohorts externallib param defintions we were mistakenly
accepting floating point values.
MDL-40666 cohort: remove duplicated code from tests
MDL-40666 enrol|cohort: PARAM_NUMBER -> PARAM_INT

PARAM_NUMBER is a float, our id's are int's, fix the incorrect PARAM
definition.