Update to new missing-method-decl warning style

As a side-effect, the new `{-# MINIMAL #-}` facility (see #7633) changes
the existing separate warnings for each missing method declaration:

{{{
No explicit method or default declaration for ‛+’
In the instance declaration for ‛Num T’
}}}

to a single warning per class of the following form:

{{{
No explicit implementation for
  ‛+’, ‛*’, ‛abs’, ‛signum’, and ‛fromInteger’
In the instance declaration for ‛Num T’
}}}

This commit adapts the reference output files in the testsuite to follow
this change in GHC's warnings messages.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Tests for MINIMAL pragma (#7633)