Use CMAKE_BUILD_TYPE to do Asan builds

Use `cmake -D CMAKE_BUILD_TYPE=Asan` rather than manually setting
`-fsanitize=address`. This lets cmake determine the necessary compiler
and linker flags.

With UNSAFE_BUILD on, force -Wno-error. This is necessary to build
with MBEDTLS_TEST_NULL_ENTROPY.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index f09c632..8c9c9ce 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -773,7 +773,7 @@
     scripts/config.pl unset MBEDTLS_ENTROPY_NV_SEED
     scripts/config.pl unset MBEDTLS_ENTROPY_HARDWARE_ALT
     scripts/config.pl unset MBEDTLS_HAVEGE_C
-    CC=gcc cmake  -D UNSAFE_BUILD=ON -D CMAKE_C_FLAGS:String="-fsanitize=address -fno-common -O3" .
+    CC=gcc cmake -D CMAKE_BUILD_TYPE:String=Asan -D UNSAFE_BUILD=ON .
     make
 
     msg "test: MBEDTLS_TEST_NULL_ENTROPY - main suites (inc. selftests) (ASan build)"