test(format): keep the expansion signature off the EOF comment newline

An unterminated block comment at the end of a file swallows the
formatter's mandatory final newline, an artifact the token view already
documents as layout; the preproc signature entry now trims it so the
invariant stays about tokens.  The triggering input joins the corpus.

Assisted-by: GLM 5.3

format/testdata/fuzz/FuzzFormatExpansionRoundTrip/062b12d69fb9426b

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#/*0")