net: don't check "invalid.invalid" lookup errors in TestLookupHostCancel

The exact error isn't actually relevant to the test,
and may depend on whether the Go or cgo resolver is used.

Also run the test in parallel, because it spends most of its time
sleeping in between lookups.

Fixes #38767
Fixes #43140

Change-Id: I2d64ffddf2eb114a69ed3242daa9a9e4a5679f67
Reviewed-on: https://go-review.googlesource.com/c/go/+/369037


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>