Allow for using regex in file_must_contain #161
Description
Currently the file_must_contain verification just checks if a string exists. We should allow for using regex to specify more complex conditions. For example in #158 the verification passed but the pixi run ... string had been tacked on to the end of a comment. It would be good if I could say
^pixi run ...
to specify that the string should exist, and that it should also be at the start of a line.