Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Enable SUBDIR_PARALLEL for lib/googletest

This saves a few seconds in a parallel build since we can build the
gtest_main and gmock subdirectories in parallel.

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D26760

Add glue Makefile for tests under lib/googletest/...

Unconditionally build/install libg{mock,test}_main

They are supporting libraries and as such, will need to be built/installed
when MK_TESTS == no during buildworld, i.e., the "make libraries" phase.

Otherwise, dependent components cannot rely on the libraries, like
`cddl/usr.sbin/zfsd/tests`.

Merge build glue for libraries and tests done on github

I need to doublecheck my work vs the port, but I believe that this covers the
initial integration of all upstream tests.

Ref: https://github.com/ngie-eign/freebsd/tree/googletest-integration