psa: Initialize crypto in tests that need to

Don't unconditionally enable PSA Crypto for all tests. Only enable it in
tests that require it. This allows crypto tests to check that
psa_crypto_init() fails when it is supposed to fail, since we want to
perform some action in a test, and then call psa_crypto_init() and check
the result without it having been called previously.
diff --git a/tests/suites/test_suite_pk.function b/tests/suites/test_suite_pk.function
index 05427c0..d85d9ed 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -124,6 +124,8 @@
     size_t len;
     mbedtls_pk_debug_item dbg;
 
+    TEST_ASSERT( psa_crypto_init() == 0 );
+
     mbedtls_pk_init( &pk );
     mbedtls_pk_init( &pk2 );