add tests for reading multiple CSVs with read_portfolio_csv()

[cjyetman]

No description provided.

[github-actions]

Coverage Report

file	head	main	diff
Overall	62%	62%	⬆️ 0.34%
R/canonize_path.R	91%	91%	0.0%
R/char_to_.R	0%	0%	0.0%
R/determine_header_types.R	92%	92%	0.0%
R/determine_headers.R	87%	87%	0.0%
R/get_csv_specs.R	0%	0%	0.0%
R/guess_delimiter.R	100%	100%	0.0%
R/guess_file_encoding.R	89%	89%	0.0%
R/guess_numerical_mark.R	94%	94%	0.0%
R/has_binary_null.R	100%	100%	0.0%
R/has_consistent_fields_per_line.R	100%	100%	0.0%
R/has_header.R	89%	89%	0.0%
R/has_newline_at_end.R	100%	100%	0.0%
R/is_file_accessible.R	100%	100%	0.0%
R/is_readable_file.R	100%	100%	0.0%
R/is_text_file.R	100%	100%	0.0%
R/is_valid_currency_code.R	100%	100%	0.0%
R/is_valid_cusip.R	100%	100%	0.0%
R/is_valid_isin.R	100%	100%	0.0%
R/read_first_line.R	88%	88%	0.0%
R/read_portfolio_csv.R	91%	88%	⬆️ 2.8%
R/simplify_if_one_col_df.R	100%	100%	0.0%