run-tests: explicitly declare the list of dropped environment variable It will make the list clearer and more maintainable.

test: drop usage of `HGMERGE` in `test-lfconvert.t` Using configuration instead of environment variable has a strange effect on the output, but no impact on the end result.

test: drop usage of `HGMERGE` in `test-merge-types.t` Using configuration instead of environment variable has a strange effect on the output, but no impact on the end result.

test: use the "correct" merge tool in `test-rebase-detach.t` The value of `HGMERGE` overrides the value passed through --config. We are about to drop this `HGMERGE` environment variable so we update the value used in config to match what is currently in use. This is another example of confusion introduced by the `HGMERGE` environment variable.

test: also deactivate `ui.merge` when testing merge tool We are about to introduce a default config for `ui.merge`. Since `test-merge-tools.t` is about testing merge configuration, we need to make sure it won't interfere.

tests: adjust merge tool config in test-largefiles-update.t Previously, the `ui.merge=internal:fail` configuration was shadowed by the the `HGMERGE=internal:merge` environment variable. We would like to remove this usage of `HGMERGE` variable because it is error-prone (eg: this very test). We start by cleaning up test independently to avoid large churn when the change happens.