cmd/go: fix a misleading sentence

Apologies for not reviewing the previous CL; I was on the road.
The text was misleading; execution is no more separate than with
any other test. Rewritten. Dropped the odd adverb.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8439045

src/cmd/go/doc.go

@@ -398,8 +398,8 @@ the file pattern "*_test.go".  These additional files can contain test functions
 benchmark functions, and example functions.  See 'go help testfunc' for more.
 Each listed package causes the execution of a separate test binary.
 
-Additionally, test files that declare a package with the suffix "_test" will be
-compiled and executed separately from the package test binary.
+Test files that declare a package with the suffix "_test" will be compiled as a
+separate package, and then linked and run with the main test binary.
 
 By default, go test needs no arguments.  It compiles and tests the package
 with source in the current directory, including tests, and runs the tests.

src/cmd/go/test.go

@@ -50,8 +50,8 @@ the file pattern "*_test.go".  These additional files can contain test functions
 benchmark functions, and example functions.  See 'go help testfunc' for more.
 Each listed package causes the execution of a separate test binary.
 
-Additionally, test files that declare a package with the suffix "_test" will be
-compiled and executed separately from the package test binary.
+Test files that declare a package with the suffix "_test" will be compiled as a
+separate package, and then linked and run with the main test binary.
 
 By default, go test needs no arguments.  It compiles and tests the package
 with source in the current directory, including tests, and runs the tests.