internal/lsp/tests: re-enable MultiModule tests on plan9
internal/lsp/tests: re-enable MultiModule tests on plan9

According to https://go.dev/issue/50478#issuecomment-1006947143, Plan
9 does not support renaming across directories. For the MultiModule
test cases we want to rename go.mod to testdata/go.mod, so we need to
fall back to copying if os.Rename fails.

Fixes golang/go#50478.

Change-Id: I30b7601e1d4b122ce114015f3585f6ba6d6c9714
Reviewed-on: https://go-review.googlesource.com/c/tools/+/376115
Trust: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools