Segfault on "unicode" tests with go1.7.4, bigger problems with go1.8beta2

[elliott5]

Following upstream changes to SSA, this code does not compile.

After changing lines 254/5 of interp_test.go to:

if mainPkg == nil && len(initialPkgs) == 1 {
		testmainPkg := prog.CreateTestMainPackage(initialPkgs[0])

Most tests pass using go1.7.4 on darwin, but the tests fail with a segfault running the unicode tests.

Testing with go1.8beta2 shows far bigger issues, possibly related to the os API.

Not planning to fix these issues myself, at least in the short term, I just wanted to report their existence to warn others.