net/http/internal/testcert: use FIPS-compliant certificate

Upgrade the test certificate from RSA 1024 (not FIPS-approved)
to RSA 2048 (FIPS-approved), allowing tests to pass when
the dev.boringcrypto branch FIPS-only mode is enabled.

Fixes #48674.

Change-Id: I613d2f8d0207bf3683fd0df256bf0167604996c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353869


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>