Keep code only required for testing out of the production library

Before:

$ find . -name \*.so| xargs nm|grep simple_task_
000000000004ffd0 T fellow_simple_task_run

After:

$ find . -name \*.so| xargs nm|grep simple_task_
<EOF>

Motivated by #27

src/Makefile.am

@@ -199,8 +199,6 @@ fellow_sources = \
 	fellow_diag.h \
 	fellow_errhandling.h \
 	fellow_task.h \
-	fellow_testenv.h \
-	fellow_testenv.c \
 	foreign/qdef.h \
 	foreign/vend.h \
 	foreign/ban.h
@@ -299,7 +297,10 @@ endif
 ############################################################
 ## test programs
 
-standalone_aux = aux_cache_wrk.c
+standalone_aux = \
+	aux_cache_wrk.c \
+	fellow_testenv.h \
+	fellow_testenv.c
 
 fellow_log_test_ndebug_LDADD = libfellow.la
 fellow_log_test_ndebug_CFLAGS = $(AM_CFLAGS) $(DEV_CFLAGS) $(XXHASH_CFLAGS) -DTEST_DRIVER