Add a setting to remove part of the module name

[nyurik]

In some cases, especially when the snapshot files reside in a sub-directory next to the test files, snapshot filenames become increasingly hard to manage. I would like to propose a setting to remove some beginning parts of the module name with a setting.

Example setup

My project crate myproj contains a proc macro in the crate myproj-macros. When testing proc-macros, it is the easiest to keep the tests in a sub-directory, e.g. ./myproj-macros/src/tests instead of being in parallel to ./myproj-macros/src because procmacro crates cannot have any public exported functions (this way these are unit, not integration tests, and the entire test submodule is behind #[cfg(test)]).

If my tests use insta, the snapshot dir is created next to them in the ./myproj-macros/src/tests/snapshots. The filename then becomes huge (assuming i am using named tests)

./myproj-macros/src/tests/snapshots/myproj_macros__tests__testfilename_testname.snap

A more readable name would be testfilename__testname.snap

Suggestion

The test name is generated with module_path!() -- which in my case is myproj_macros::tests::testfilename. I think it would be great if we could remove some prefix from it. Some approaches:

• give a hardcoded string myproj_macros::tests -- that string must exits in the module name and end in :: (fail otherwise)
• give a number of levels to remove, 2 in this case
• number of characters to remove (not a good idea, i know)

[max-sixty]

Yes, is this the same as #377; do we need more flexibility or is the proposed default there of no repetition sufficient?