testament: error instead of silently overwrite a spec

[timotheecour]

followup #15930
refs https://github.com/nim-lang/Nim/pull/15930/files#r531796278 /cc @xflywind

before PR, lots of tests were wrong (in the sense they assumed some things were being tested but they were in fact not being tested because testament spec was overwriting the fields written more than once)

after PR: this is fixed and overwriting would result in errors

future work

make nimout comparison use nimoutCheck instead of:

elif expected.nimout.len > 0 and expected.nimout.normalizeMsg notin given.nimout.normalizeMsg:

and then use

nimout: '''
tcaseexpr1.nim(33, 10) Error: not all cases are covered2; missing: {C}
tcaseexpr1.nim(39, 12) Error: type mismatch: got <string> but expected 'int literal(10)'
'''

in tests/casestmt/tcaseexpr1.nim

CI failure unrelated: #16169

[ringabout]

looks good to me

[timotheecour]

@Araq I'm merging this