test_program_read_small_data_sizes: get erase size specific to the sector we use

The test case only uses one specific sector, but the erase size
is obtained for the whole block device instead. This doesn't work
if different regions of the flash don't have a common erase size.

Fix the issue by getting the erase size at the address we use.

Author: LDong-Arm

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

@@ -584,7 +584,6 @@ void test_program_read_small_data_sizes()
 
     TEST_SKIP_UNLESS_MESSAGE(block_device != NULL, "no block device found.");
 
-    bd_size_t erase_size = block_device->get_erase_size();
     bd_size_t program_size = block_device->get_program_size();
     bd_size_t read_size = block_device->get_read_size();
     TEST_ASSERT(program_size > 0);
@@ -606,6 +605,7 @@ void test_program_read_small_data_sizes()
 
     // Determine starting address
     bd_addr_t start_address = 0;
+    bd_size_t erase_size = block_device->get_erase_size(start_address);
 
     for (int i = 1; i <= 7; i++) {
         err = buff_block_device->erase(start_address, erase_size);