The wrong value was passed to `ExUnit.Callbacks.raise_merge_reserved!/3`,
which caused the current value to be used in the error message instead of
the one passed in the context. With a setup block like this:
setup do
{:ok, file: "foo"}
end
The error message uses the current `:file` value (the path to the
test file) instead of the passed one ("foo"):
** (RuntimeError) ExUnit callback in MyAppTest is trying to
set reserved field :file to "/path/to/test/my_app_test.exs"
This one-character patch uses the passed value instead, resulting
in an error message like this:
** (RuntimeError) ExUnit callback in MyAppTest is trying to
set reserved field :file to "foo"