Use native Ginkgo test runner instead of cmd/e2e.

This commit deletes cmd/e2e and updates hack/ginkgo-e2e.sh to use the
'ginkgo' command instead. All logic from cmd/e2e/e2e.go and
test/e2e/driver.go have been combined into the new file
test/e2e/e2e_test.go.

Additionally, several tests which made poor assumptions about cwd or
used testContext before it was set have been fixed.

This change is generally intended to have no externally visible changes,
aside from the following caveats:
- The -t/--tests flag has been removed
- Calling cmd/e2e/e2e directly obviously won't work, but that was never
  supported anyway
- If the GINKGO_PARALLEL environment variable is set to y, then ginkgo
  will run test specs in parallel. (Currently defaults to n, since some
  tests are broken in this mode.)

hack/lib/golang.sh

@@ -44,7 +44,6 @@ readonly KUBE_CLIENT_BINARIES_WIN=("${KUBE_CLIENT_BINARIES[@]/%/.exe}")
 
 # The set of test targets that we are building for all platforms
 readonly KUBE_TEST_TARGETS=(
-  cmd/e2e
   cmd/integration
   cmd/gendocs
   cmd/genman
@@ -59,6 +58,7 @@ readonly KUBE_TEST_PORTABLE=(
   hack/e2e-suite
   hack/e2e-internal
   hack/ginkgo-e2e.sh
+  hack/lib
 )
 
 # If we update this we need to also update the set of golang compilers we build