bignum_core.py: Add "BignumCoreShiftL()"

This patch introduces automatic test input generation for
`mpi_core_shift_l()` function.

It also adds two utility functions in bignum_common.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>

scripts/mbedtls_dev/bignum_common.py

@@ -80,6 +80,23 @@ def hex_digits_for_limb(limbs: int, bits_in_limb: int) -> int:
     """ Retrun the hex digits need for a number of limbs. """
     return 2 * (limbs * bits_in_limb // 8)
 
+def hex_digits_max_int(val: str, bits_in_limb: int) -> int:
+    """ Return the first number exceeding maximum  the limb space
+    required to store the input hex-string value. This method
+    weights on the input str_len rather than numerical value
+    and works with zero-padded inputs"""
+    n = ((1 << (len(val) * 4)) - 1)
+    l = limbs_mpi(n, bits_in_limb)
+    return bound_mpi_limbs(l, bits_in_limb)
+
+def zfill_match(reference: str, target: str) -> str:
+    """ Zero pad target hex-string the match the limb size of
+    the refference input """
+    lt = len(target)
+    lr = len(reference)
+    targen_len = lr if lt < lr else lt
+    return "{:x}".format(int(target, 16)).zfill(targen_len)
+
 class OperationCommon(test_data_generation.BaseTest):
     """Common features for bignum binary operations.