Fix test-at-point path for a symlinked project root

[bbatsov]

projectile-compilation-dir resolves symlinks and buffer-file-name doesn't, so a project reached through a symlinked path made the two disagree about where the root is. The relative name then climbed out of the project and the test runner rejected it:

go test -run '^TestApplyDiscount$' ./../../../../../tmp/checkout/pricing
directory /tmp/checkout/pricing outside main module

Every rule builds its file argument the same way, so this isn't Go-specific. Anything under /tmp on macOS hits it, as does a symlinked ~/src.

The name is still taken as spelled first. That's what keeps a file under a symlinked subdirectory working, since resolving that one is what would send it out of the project. Only when the spelled name escapes are both sides resolved, and that result is used only if it stays inside.

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