Fix loop variable captured by func literal

[peczenyj]

Description

by running go vet ./... with go latest version there are some warning about loop variable captured by func literal

this is linked to

https://github.com/golang/go/wiki/CommonMistakes#using-reference-to-loop-iterator-variable

and

https://github.com/golang/go/wiki/LoopvarExperiment

$ go version
go version go1.21.1 linux/amd64

$ go vet ./...
# github.com/gofiber/fiber/v2/internal/go-ole
internal/go-ole/variant.go:15:21: possible misuse of unsafe.Pointer
internal/go-ole/variant.go:23:22: possible misuse of unsafe.Pointer
internal/go-ole/variant.go:33:42: possible misuse of unsafe.Pointer
# github.com/gofiber/fiber/v2/middleware/compress
middleware/compress/compress_test.go:60:30: loop variable level captured by func literal
# github.com/gofiber/fiber/v2/middleware/filesystem
middleware/filesystem/filesystem_test.go:124:63: loop variable tt captured by func literal
middleware/filesystem/filesystem_test.go:126:25: loop variable tt captured by func literal
middleware/filesystem/filesystem_test.go:128:7: loop variable tt captured by func literal
middleware/filesystem/filesystem_test.go:130:26: loop variable tt captured by func literal
# github.com/gofiber/fiber/v2/middleware/pprof
middleware/pprof/pprof_test.go:109:32: loop variable sub captured by func literal
middleware/pprof/pprof_test.go:110:7: loop variable sub captured by func literal
middleware/pprof/pprof_test.go:138:48: loop variable sub captured by func literal
middleware/pprof/pprof_test.go:139:7: loop variable sub captured by func literal

I have no idea how to fix the possible misuse of unsafe.Pointer (this can be a false positive, someone with experience in windows and go unsafe must check) but the loop variable is easier

However this trigger some random 500 errors in the middleware/pprof because some handlers are not supposed to be called more than once at same time.

We can see outputs like

Could not enable tracing: runtime error: tracing is already enabled

or

Could not enable CPU profiling: cpu profiling already in use

I remove the t.Parallel() of such tests

Type of change

minor change in tests

Checklist:

• [ x] For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] I have made corresponding changes to the documentation - /docs/ directory for https://docs.gofiber.io/
• [ x] I have added tests that prove my fix is effective or that my feature works
• [ x] New and existing unit tests pass locally with my changes
• [ x] If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)
• [ x] I tried to make my code as fast as possible with as few allocations as possible
• [ x] For new code I have written benchmarks so that they can be analyzed and improved

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