Reword test cases

Reword test cases to be easier to read and understand.
Adds comments to better explain what the test is doing.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index 1808233..1c0661f 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -694,22 +694,22 @@
 Base test mbedtls_mpi_exp_mod #6 (Negative base + exponent)
 mbedtls_mpi_exp_mod:10:"-23":10:"-13":10:"29":10:"":10:"0":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
-Base test mbedtls_mpi_exp_mod #7 (MAX_SIZE exponent)
+Test mbedtls_mpi_exp_mod: MAX_SIZE exponent
 mbedtls_mpi_exp_mod_size:2:MBEDTLS_MPI_MAX_SIZE:2:10:"":0
 
-Base test mbedtls_mpi_exp_mod #8 (MAX_SIZE + 1 exponent)
+Test mbedtls_mpi_exp_mod: MAX_SIZE + 1 exponent
 mbedtls_mpi_exp_mod_size:2:MBEDTLS_MPI_MAX_SIZE + 1:2:10:"":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
-Base test mbedtls_mpi_exp_mod #9 (MAX_SIZE modulus)
+Test mbedtls_mpi_exp_mod: MAX_SIZE modulus
 mbedtls_mpi_exp_mod_size:2:2:MBEDTLS_MPI_MAX_SIZE:10:"":0
 
-Base test mbedtls_mpi_exp_mod #10 (MAX_SIZE + 1 modulus)
+Test mbedtls_mpi_exp_mod: MAX_SIZE + 1 modulus
 mbedtls_mpi_exp_mod_size:2:2:MBEDTLS_MPI_MAX_SIZE + 1:10:"":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
-Base test mbedtls_mpi_exp_mod #11 (MAX_SIZE exponent and modulus)
+Test mbedtls_mpi_exp_mod: MAX_SIZE exponent and modulus
 mbedtls_mpi_exp_mod_size:2:MBEDTLS_MPI_MAX_SIZE:MBEDTLS_MPI_MAX_SIZE:10:"":0
 
-Base test mbedtls_mpi_exp_mod #12 (MAX_SIZE + 1 exponent and modulus)
+Test mbedtls_mpi_exp_mod: MAX_SIZE + 1 exponent and modulus
 mbedtls_mpi_exp_mod_size:2:MBEDTLS_MPI_MAX_SIZE + 1:MBEDTLS_MPI_MAX_SIZE + 1:10:"":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
 Test mbedtls_mpi_exp_mod #1