x/tools/gopls/internal/lsp/mod: TestModfileRemainsUnchanged fails when run as a dependency of another module

[bcmills]

What version of Go are you using (go version)?

$ go version
go version devel go1.20-245e95dfab Wed Jan 11 22:29:34 2023 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/usr/local/google/home/bcmills/.cache/go-build"
GOENV="/usr/local/google/home/bcmills/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/tmp/tmp.uw94Sef663/.gopath/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/tmp/tmp.uw94Sef663/.gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/google/home/bcmills/sdk/gotip"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/google/home/bcmills/sdk/gotip/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel go1.20-245e95dfab Wed Jan 11 22:29:34 2023 +0000"
GCCGO="/usr/local/google/home/bcmills/bin/gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="c++"
CGO_ENABLED="1"
GOMOD="/tmp/tmp.uw94Sef663/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build227727649=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ go mod init example
$ go get golang.org/x/tools/go/expect@HEAD golang.org/x/tools/gopls/internal/lsp/mod@HEAD
$ go test golang.org/x/tools/gopls/internal/lsp/mod

What did you expect to see?

Passing tests.

What did you see instead?

--- FAIL: TestModfileRemainsUnchanged (0.00s)
    mod_test.go:37: stat testdata/unchanged: no such file or directory
FAIL
FAIL    golang.org/x/tools/gopls/internal/lsp/mod       0.048s
FAIL

The problem is that the testdata directory contains a go.mod file, which causes it not to be included in the golang.org/x/tools module containing the actual test (see #27852).

[gopherbot]

Change https://go.dev/cl/518976 mentions this issue: gopls/internal/lsp/mod: remove TestModfileRemainsUnchanged