Fix restart behavior with circular vocabs, and add a test for this

core/vocabs/loader/loader-tests.factor

@@ -171,3 +171,11 @@ forget-junk
 ] with-compilation-unit
 
 [ ] [ "vocabs.loader.test.h" require ] unit-test
+
+
+[
+    "vocabs.loader.test.j" forget-vocab
+    "vocabs.loader.test.k" forget-vocab
+] with-compilation-unit
+
+[ ] [ [ "vocabs.loader.test.j" require ] [ drop :1 ] recover ] unit-test

core/vocabs/loader/loader.factor

@@ -65,6 +65,7 @@ ERROR: circular-dependency name ;
     [
         +parsing+ >>source-loaded?
         dup vocab-source-path [ parse-file ] [ [ ] ] if*
+        [ +parsing+ >>source-loaded? ] dip
         [ % ] [ assert-depth ] if-bootstrapping
         +done+ >>source-loaded? drop
     ] [ ] [ f >>source-loaded? ] cleanup ;

core/vocabs/loader/test/j/j.factor

@@ -0,0 +1,2 @@
+IN: vocabs.loader.test.j
+"vocabs.loader.test.k" require

core/vocabs/loader/test/j/tags.txt

@@ -0,0 +1 @@
+unportable

core/vocabs/loader/test/k/k.factor

@@ -0,0 +1,2 @@
+IN: vocabs.loader.test.k
+USE: vocabs.loader.test.j

core/vocabs/loader/test/k/tags.txt

@@ -0,0 +1 @@
+unportable