net/http/httputil: reduced log verbosity in reverseproxy_test.go
net/http/httputil: reduced log verbosity in reverseproxy_test.go

For functions TestClonesRequestHeaders and TestReverseProxy_PanicBodyError,
I made changes to update the log verbosity.

Fixes #25634

Change-Id: I2a0ef70a8191cfb1a0005949345be722fb4ab62e
Reviewed-on: https://go-review.googlesource.com/115296
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,josharian/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,josharian/go,golang/go,golang/go,josharian/go,golang/go,CAFxX/go,nawawi/go,akutz/go,golang/go,cloudflare/go,golang/go,cloudflare/go,golang/go,akutz/go,akutz/go,akutz/go,akutz/go