Refactor src/parser/tests/parser_tests.rs into seperate modules

[riederm]

Describe the bug
The file src/parser/tests/parser_tests.rs has ~1500 lines and contains tests concerning mulitple parse-topcis raning from Container-Parse-tests, Statement-parse-tests to parse-error-tests

It is hard to find the appropriate location for new tests and to determine whether there are tests missing because its not clear where to look for them?

refactor this file into mulitple modules with a cohesive test-topic like:

• container-parse-tests
• statement-parse-tests
• parse-error-tests (should be moved to appropriate files in parse_errors-folder)
• etc.