Reduce number of threades in block device test

In addition, prevent FS tests from running on internal flash,
due to the fact that file system on internal flash is not part of
our offering (TDBStore should be used there instead).

Author: offirko

features/storage/TESTS/blockdevice/general_block_device/main.cpp

@@ -51,7 +51,7 @@ using namespace utest::v1;
 
 #define TEST_BLOCK_COUNT 10
 #define TEST_ERROR_MASK 16
-#define TEST_NUM_OF_THREADS 5
+#define TEST_NUM_OF_THREADS 4
 
 const struct {
     const char *name;