test: use T.TempDir instead of MkdirTemp

[alexandear]

From the doc:

TempDir returns a temporary directory for the test to use. The directory is automatically removed by Cleanup when the test and all its subtests complete

[erikdubbelboer]

Seems like it isn't working on windows.

[alexandear]

T.TempDir works on Windows. Seems something wrong with the test or tested code itself.

I reverted PR's changes, replaced defer os.RemoveAll(tempdir) in TestServeFileSmallNoReadFrom with:

defer func() {
	if err := os.RemoveAll(tempdir); err != nil {
		t.Log(err)
	}
}()

run the test on Windows and got the same error as we see in the CI:

go test -v -run TestServeFileSmallNoReadFrom
=== RUN   TestServeFileSmallNoReadFrom
=== PAUSE TestServeFileSmallNoReadFrom
=== CONT  TestServeFileSmallNoReadFrom
    fs_test.go:163: remove C:\Users\builder\AppData\Local\Temp\3\httpexpect1148472821\hello: The process cannot access the file because it is being used by another process.
--- FAIL: TestServeFileSmallNoReadFrom (0.06s)
FAIL
exit status 1
FAIL    github.com/valyala/fasthttp     0.408s

It's needed to investigate further.

[alexandear]

@erikdubbelboer fixed it, please review. The problem was that the code doesn't close a file handler. And Windows can't delete a folder with opened files in it.

[erikdubbelboer]

Thanks!