patch 8.1.1838: there is :spellwrong and :spellgood but not :spellrare

Problem:    There is :spellwrong and :spellgood but not :spellrare.
Solution:   Add :spellrare. (Martin Tournoij, closes #4291)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index b3abcba..131ba01 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -236,6 +236,7 @@
 	test_source \
 	test_source_utf8 \
 	test_spell \
+	test_spellfile \
 	test_startup \
 	test_startup_utf8 \
 	test_stat \
@@ -411,6 +412,7 @@
 	test_sound.res \
 	test_source.res \
 	test_spell.res \
+	test_spellfile.res \
 	test_startup.res \
 	test_stat.res \
 	test_substitute.res \