fix: potential panic in validate

[markphelps]

• fixes case where finding pos of error could panic
• moves cue/fixtures to cue/testdata
• adds fuzz tests for cue.Validate
• fixed some linter warnings about deprecated ioutil package

Before Fix

--- FAIL: FuzzValidate (97.59s)
    --- FAIL: FuzzValidate (0.00s)
        testing.go:1485: panic: runtime error: index out of range [-1]
            goroutine 29131 [running]:
            runtime/debug.Stack()
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/runtime/debug/stack.go:24 +0xbc
            testing.tRunner.func1()
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1485 +0x264
            panic({0x102b97700, 0x1400a1d8918})
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/runtime/panic.go:884 +0x204
            go.flipt.io/flipt/internal/cue.FeaturesValidator.Validate({0x1400a8024a0?, {0x1400a8024a0?, 0x1400a80f3b0?, 0x0?}}, {0x1029f30ca, 0x3}, {0x14009640050?, 0x0?, 0x4?})
            	/Users/markphelps/workspace/flipt/internal/cue/validate.go:76 +0x484
            go.flipt.io/flipt/internal/cue.FuzzValidate.func1(0x1400a7f8340, {0x14009640050, 0x6, 0x10})
            	/Users/markphelps/workspace/flipt/internal/cue/validate_fuzz_test.go:26 +0x10c
            reflect.Value.call({0x102b24a00?, 0x102bcad28?, 0x1400c914e38?}, {0x1029f334d, 0x4}, {0x1400a783080, 0x2, 0x0?})
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/reflect/value.go:586 +0x87c
            reflect.Value.Call({0x102b24a00?, 0x102bcad28?, 0x14000000000?}, {0x1400a783080?, 0x102bc9680?, 0x102e46ab0?})
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/reflect/value.go:370 +0x90
            testing.(*F).Fuzz.func1.1(0x1400c90a000?)
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/fuzz.go:335 +0x360
            testing.tRunner(0x1400a7f8340, 0x1400a80f290)
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1576 +0x10c
            created by testing.(*F).Fuzz.func1
            	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/fuzz.go:322 +0x4c4


    Failing input written to testdata/fuzz/FuzzValidate/9d39dbf6febda3de

[codecov]

Codecov Report

Merging #1831 (f9f702d) into main (c846fc3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1831   +/-   ##
=======================================
  Coverage   71.63%   71.64%           
=======================================
  Files          58       58           
  Lines        5500     5501    +1     
=======================================
+ Hits         3940     3941    +1     
  Misses       1335     1335           
  Partials      225      225           

Impacted Files	Coverage Δ
internal/cue/validate.go	84.61% <100.00%> (+0.40%)	⬆️

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[yquansah]

LGTM