cmd/internal/testdir: Test/fixedbugs/bug513.go fails on FreeBSD 15.0

Now that issue #77411 is resolved at tip, the gotip-freebsd-amd64_15.0 builder has only one remaining failing test:

=== RUN   Test/fixedbugs/bug513.go
=== PAUSE Test/fixedbugs/bug513.go
=== CONT  Test/fixedbugs/bug513.go
    testdir_test.go:147: output should be empty when (optional) expected-output file fixedbugs/bug513.out is not present. Instead saw
        This sanitizer is not compatible with enabled ASLR and binaries compiled with PIE
        
--- FAIL: Test/fixedbugs/bug513.go (38.65s)

Once the failure is fixed or test skipped, the builder's known issue can be removed. This is the tracking issue for that. (Issues #65425 and #73788 may be related.)

CC @golang/freebsd.