os,x/website/internal/web: TestTypeScript failures due to mismatched separators in error strings

[gopherbot]

#!watchflakes
post <- pkg == "golang.org/x/website/internal/web" && test == "TestTypeScript"

Issue created automatically to collect these failures.

Example (log):

--- FAIL: TestTypeScript (0.07s)
    --- FAIL: TestTypeScript/file_not_found (0.01s)
        site_test.go:120: ServeHTTP() mismatch (-want +got):
              strings.Join({
              	"\n\nopen testdata",
            - 	"/",
            + 	`\`,
              	"notfound.ts: The system cannot find the file specified.\n",
              }, "")

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- pkg == "golang.org/x/website/internal/web" && test == "TestTypeScript"

2022-09-27 21:05 windows-arm64-10 website@cf701bc4 go@18888751 x/website/internal/web.TestTypeScript (log)

--- FAIL: TestTypeScript (0.07s)
    --- FAIL: TestTypeScript/file_not_found (0.01s)
        site_test.go:120: ServeHTTP() mismatch (-want +got):
              strings.Join({
              	"\n\nopen testdata",
            - 	"/",
            + 	`\`,
              	"notfound.ts: The system cannot find the file specified.\n",
              }, "")

— watchflakes

[bcmills]

This is fallout from https://go.dev/cl/426094; fix under discussion in https://go.dev/cl/437976.

[bcmills]

I expect that the fix may involve changes in both the way os.DirFS reports errors and the specific errors that TestTypeScript expects to see.

[gopherbot]

Change https://go.dev/cl/437976 mentions this issue: os: if dirFS.Open fails, undo use of backslashes in error message

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- pkg == "golang.org/x/website/internal/web" && test == "TestTypeScript"

2022-10-04 18:01 windows-arm64-10 website@f3791178 go@c318f191 x/website/internal/web.TestTypeScript (log)

--- FAIL: TestTypeScript (0.21s)
    --- FAIL: TestTypeScript/file_not_found (0.01s)
        site_test.go:120: ServeHTTP() mismatch (-want +got):
              strings.Join({
              	"\n\nopen testdata",
            - 	"/",
            + 	`\`,
              	"notfound.ts: The system cannot find the file specified.\n",
              }, "")

— watchflakes