Tests: provide necessary functions for MBEDTLS_PSA_INJECT_ENTROPY

The build option MBEDTLS_PSA_INJECT_ENTROPY requires some extra platform
functions, for historical reasons. To enable us to test this option, provide
a version of these functions for testing.

(These versions would actually work in production, but providing them in the
library in a way that doesn't break existing users might be slightly tricky,
so it's out of scope of this commit.)

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/config.py b/scripts/config.py
index ac5f77c..e9d4218 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -208,7 +208,7 @@
     'MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG', # behavior change + build dependency
     'MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER', # incompatible with USE_PSA_CRYPTO
     'MBEDTLS_PSA_CRYPTO_SPM', # platform dependency (PSA SPM)
-    'MBEDTLS_PSA_INJECT_ENTROPY', # build dependency (hook functions)
+    'MBEDTLS_PSA_INJECT_ENTROPY', # conflicts with platform entropy sources
     'MBEDTLS_RSA_NO_CRT', # influences the use of RSA in X.509 and TLS
     'MBEDTLS_SHA256_USE_A64_CRYPTO_ONLY', # interacts with *_USE_A64_CRYPTO_IF_PRESENT
     'MBEDTLS_SHA512_USE_A64_CRYPTO_ONLY', # interacts with *_USE_A64_CRYPTO_IF_PRESENT