use insta for witness tests

[suaviloquence]

See also #933 (comment).

Replaces custom snapshot code with insta when testing witness generation for lints, which should have better contributor experience.

[suaviloquence]

It's still ron, so multiline strings will not be rendered super well. We could use a different serializer like yaml if this becomes a problem, and it would be very easy to change.

[obi1kenobi]

yaml is an interesting idea. If you try it and think it looks good, do you mind switching this to it?

I suspect many of our witnesses will be multi-line once we get past the basic "item deleted" ones, and diffing multi-line strings represented with escape sequences is going to get really annoying quickly. We risk letting bugs slip just because it was hard to notice the snapshot changed in an undesirable way (e.g. bad indentation etc.)

[suaviloquence]

It looks like the yaml serializer that insta uses doesn't support multiline strings (even though yaml-the-langauge does...) mitsuhiko/insta#372. However toml handles multiline strings perfectly, my test code is below (i added extra lines to the witness hint)

---
source: src/query.rs
description: "Lint `function_missing` did not have the expected witness output.\nSee https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md#testing-witnesses\nfor more information."
expression: "&actual_witnesses"
---
[["./test_crates/feature_flags_validation/"]]
filename = 'src/lib.rs'
begin_line = 4
hint = '''
line 1
line 2
feature_flags_validation::foo_becomes_gated(...);'''

[["./test_crates/feature_flags_validation/"]]
filename = 'src/lib.rs'
begin_line = 5
hint = '''
line 1
line 2
feature_flags_validation::bar_becomes_gated(...);'''

[["./test_crates/features_simple/"]]
filename = 'src/lib.rs'
begin_line = 2
hint = '''
line 1
line 2
features_simple::feature_dependent_function(...);'''

[["./test_crates/function_const_removed/"]]
filename = 'src/lib.rs'
begin_line = 5
hint = '''
line 1
line 2
function_const_removed::fn_removed(...);'''

[["./test_crates/function_feature_changed/"]]
filename = 'src/lib.rs'
begin_line = 2
hint = '''
line 1
line 2
function_feature_changed::moving_from_feature_A_to_feature_B(...);'''

[["./test_crates/function_missing/"]]
filename = 'src/lib.rs'
begin_line = 1
hint = '''
line 1
line 2
function_missing::will_be_removed_fn(...);'''

[["./test_crates/function_missing/"]]
filename = 'src/lib.rs'
begin_line = 4
hint = '''
line 1
line 2
function_missing::pub_use_removed_fn(...);'''

[obi1kenobi]

Awesome work! 🚀