Makes basic-build-test.sh tests more consistent

This contains two fixes:
 * CFLAGS symbol wasn't being exported so wasn't being used in the build
 * Absence of a clean build meant the build could be made with existing
   object code that may not have code coverage instrumentation
diff --git a/tests/scripts/basic-build-test.sh b/tests/scripts/basic-build-test.sh
index 06c2eb9..f1b36c3 100755
--- a/tests/scripts/basic-build-test.sh
+++ b/tests/scripts/basic-build-test.sh
@@ -36,7 +36,8 @@
 
 
 # Step 1 - Make and instrumented build for code coverage
-CFLAGS=' --coverage -g3 -O0 '
+export CFLAGS=' --coverage -g3 -O0 '
+make clean
 make