When GOROOT_FINAL is set when running all.bash, the tests are run
before the files are copied to GOROOT_FINAL.  The tests are run with
GOROOT set, so most work fine.  This fixes two cases that do not.

In cmd/go/go_test.go we were explicitly removing GOROOT from the
environment, causing tests that did not themselves explicitly set
GOROOT to fail.  There was no need to explicitly remove GOROOT, so
don't do it.  If people choose to run "go test cmd/go" with a bad
GOROOT, that is their own lookout.

In the runtime GDB test, the linker has told gdb to find the support
script in GOROOT_FINAL, which will fail.  Check for that case, and
skip the test when we see it.

Fixes #11652.

Change-Id: I4d3a32311e3973c30fd8a79551aaeab6789d0451
Reviewed-on: https://go-review.googlesource.com/12021
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>