t7800: "defaults" is no longer a builtin tool name

073678b8e6324a155fa99f40eee0637941a70a34 reworked the
mergetools/ directory so that every file corresponds to a
difftool-supported tool.  When this happened the "defaults"
file went away as it was no longer needed by mergetool--lib.

t7800 tests that configured commands can override builtins,
but this test was not adjusted when the "defaults" file was
removed because the test continued to pass.

Adjust the test to use the everlasting "vimdiff" tool name
instead of "defaults" so that it correctly tests against a tool
that is known by mergetool--lib.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>