Parser: Test for exact missing ':' location column

This ensures compiler points to the right place when claiming that ':' is
missing.

tests/parser/tinvcolonlocation3.nim

@@ -0,0 +1,12 @@
+discard """
+  file: "tinvcolonlocation3.nim"
+  line: 12
+  column: 3
+  errormsg: "':' expected"
+"""
+try:
+  echo "try"
+except:
+  echo "except"
+finally #<- missing ':'
+  echo "finally"