internal/source: move some tests into another package

The goal of this CL is to remove the transitive test of cmd/pkgsite on
the github.com/google/go-replayers/httpreplay package. The dependency
happens through the tests of the source package, so move the test out
of the source package into its own package.

For golang/go#61399

Change-Id: Ia0d5e4310e25ec0a034cd84f0d5cbcb06b5a75d8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/538767
kokoro-CI: kokoro <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: matloob