test: link to correct library in Rust tests

Coincidentally, default GitHub CI environment contains
libsqlite.so library in place, which hid the fact that
Rust tests fail to prefer the locally built library.
The tests were previously effectively verifying that
the library from system path is correct, which is also
nice, but not the desired effect. The amended rusttest
command correctly prefers locally build libraries.

Makefile.in

@@ -1299,7 +1299,7 @@ quicktest:	./testfixture$(TEXE)
 	./testfixture$(TEXE) $(TOP)/test/extraquick.test $(TESTOPTS)
 
 rusttest:	sqlite3.h libsqlite3.la
-	( cd test/rust_suite; cargo test )
+	( cd test/rust_suite; LD_LIBRARY_PATH=../../.libs cargo test )
 
 # This is the common case.  Run many tests that do not take too long,
 # including fuzzcheck, sqlite3_analyzer, and sqldiff tests.