testing: new T.TempDir doesn't work with subtests

[bradfitz]

Using TempDir with subtests with the unreleased testing.TB.TempDir (#35998) doesn't work, as ioutil.TempDir doesn't want a slash in its pattern:

=== RUN   TestLinkerGC/only_conn
    TestLinkerGC/only_conn: testing.go:819: TempDir: pattern contains path separator
=== RUN   TestLinkerGC/only_client
    TestLinkerGC/only_client: testing.go:819: TempDir: pattern contains path separator
=== RUN   TestLinkerGC/only_server
    TestLinkerGC/only_server: testing.go:819: TempDir: pattern contains path separator
--- FAIL: TestLinkerGC (0.00s)

Seems easy enough to fix... just mangle the test name a bit.

/cc @ianlancetaylor @bcmills

[gopherbot]

Change https://golang.org/cl/229399 mentions this issue: testing: make TempDir work for subtests

[gopherbot]

Change https://golang.org/cl/229837 mentions this issue: testing: replace GOOS-specific path separator in TempDir