meson: Make test output much more useful on failure (both in CI and locally)

Applying: Address feedback on changed pg_regress output
Using index info to reconstruct a base tree...
M	src/test/regress/pg_regress.c
Falling back to patching base and 3-way merge...
Auto-merging src/test/regress/pg_regress.c
CONFLICT (content): Merge conflict in src/test/regress/pg_regress.c
Patch failed at 0001 Address feedback on changed pg_regress output

error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"