Fix failing test for missing required prop 'fields'
Fix failing test for missing required prop 'fields'

The error message has changed since this test has been
implemented.
Using sinon.match for more robustness, it asserts that
the error message is a string and that the expectation
is a substring.

JeResilieMonContrat-com/vuetable-2,ratiw/vuetable-2,c0defre4k/vuetable-2,c0defre4k/vuetable-2,ratiw/vuetable-2,JeResilieMonContrat-com/vuetable-2