pytest: update of README.md (addressed feedback, fixed typos and added custom options)

```

The tests rely on references which need to be generated upfront using reference binaries.

When the reference binaries are named `IVAS_cod_ref(.exe)` and `IVAS_dec_ref.(exe)`, pytest will find and use them.

When the reference binaries are named `IVAS_cod_ref(.exe)` and `IVAS_dec_ref(.exe)`, pytest will find and use them.

When the reference binaries are named differently, you need to specify them via the `--ref_encoder_path` and `--ref_decoder_path` options.

The tests will used the binaries `IVAS_cod(.exe)` and `IVAS_dec.(exe)` for testing. Please make sure that the binaries have been built before running the tests.

The tests will used the binaries `IVAS_cod(.exe)` and `IVAS_dec(.exe)` for testing. Please make sure that the binaries have been built before running the tests.

When different test binaries are to be used, they can be specified via the `--dut_encoder_path` and `--dut_decoder_path` options.

(DUT: Device Under Test)

```bash

# create references

# the following binaies need to be present:

# the following binaries need to be present:

# - IVAS_cod(.exe)

# - IVAS_dec.(exe)

# - IVAS_dec(.exe)

# - IVAS_cod_ref(.exe)

# - IVAS_dec_ref.(exe)

# - IVAS_dec_ref(.exe)

# pytest command lines to be executed from project root folder:

pytest tests -n auto --update_ref 1 -m create_ref

pytest tests -n auto --update_ref 1 -m create_ref_part2

Commonly used options like `-n auto` can be added to addopts within the [pytest] section in `pytest.ini`. This saves some typing when calling `pytest`.

The `-v` (or `--verbose`) option is usually helpful to see what is going on. Therefore, `-v` is currently part of addopts in `pytest.ini`. If you don't like this verbosity, you can specify the `-q` (`--quiet`) option when running `pytest`.

## Custom options

`Note:`

The custom options are listed as part of the pytest help `pytest -h`.

```text

--update_ref=UPDATE_REF              Indicate whether references shall be updated.

                                     0: Only DUT processing, no reference generation, references need to be present.

                                     1: Only reference generation (unconditionally), no DUT processing.

                                     2: DUT processing, references are generated when not present (not supported by all tests).

--dut_encoder_path=DUT_ENCODER_PATH  If specified, use given binary as DUT encoder.

--dut_decoder_path=DUT_DECODER_PATH  If specified, use given binary as DUT decoder.

--ref_encoder_path=REF_ENCODER_PATH  If specified, use given binary as REF encoder.

--ref_decoder_path=REF_DECODER_PATH  If specified, use given binary as REF decoder.

--test_vector_path=TEST_VECTOR_PATH  If specified, use given directory as base directory for test vector files.

--reference_path=REFERENCE_PATH      If specified, use given directory as base directory for reference files.

--dut_base_path=DUT_BASE_PATH        If specified, use given directory as base data directory for dut files.

--param_file=PARAM_FILE              If specified, use given param file in test_param_file.

```