Automatic merge from submit-queue

pkg/probe/http: don't compare error strings in tests

TestHTTPProbeChecker fails on the Go1.7 release candidates. The
package's history show that this was the case for Go1.5 and Go1.6
as well.

The test depend on errors holding specific string values, behavior
not guarenteed in the standard library API, and causing new test
failures every minor Go release. Just look for an error rather than
trying to inspect it using string comparison. If we feel this
impacts coverage we can add more test cases.

Fixes #15952