Extra test for optional space after fil...l (see #38)

Note that TeX searches for more "l" by repeatedly calling scan_keyword
so "fil \space\space l" is valid and TeX continues deleting spaces
after the last "l" it found (searching for more "l").  To avoid this
and really test for the <optional space> we do "fill\noexpand\space":
then when looking for more "l", TeX finds a \notexpanded:\space, and
when looking for the <optional space> TeX expands and deletes \space.

testfiles/unravel009.lvt

@@ -28,6 +28,8 @@
     \tl_if_eq:NNTF \l_tmpa_tl \l_tmpb_tl { \TRUE } { \ERROR }
     \unravel_get:nnN { mute } { \afterassignment \token_to_str:N \l_tmpa_dim = 1 pz \c_space_tl } \l_tmpb_tl
     \str_if_eq:eeTF { \l_tmpb_tl } { pz ~ } { \TRUE } { \ERROR }
+    \unravel_get:nnN { mute } { \afterassignment \token_to_str:N \l_tmpa_skip = 1pt minus 3filll \exp_not:N \c_space_tl \BOOM } \l_tmpb_tl
+    \tl_if_eq:NNTF \l_tmpa_tl \l_tmpb_tl { \TRUE } { \ERROR }
     %
     \unravel_get:nnN { mute } { \exp_last_unbraced:Nx \show {\if_dim:w 3pt>2pt\else:\fi:} } \l_tmpa_tl
     \tl_if_empty:NTF \l_tmpa_tl { \TRUE } { \ERROR }

testfiles/unravel009.tlg

@@ -35,6 +35,7 @@ TeX's advice is:
     Chapter 27 of The TeXbook.)
 Defining \g__unravel_input_15_gtl on line ...
 TRUE
+TRUE
 > \relax=\relax.
 <argument> \relax 
 l. ...  }