go/loader: fix broken tests after context removal from testing package

Updates golang/go#11811
Updates golang/go#18199

Change-Id: I2ce4615653034563a64b9c126651d2a6ce2aef50
Reviewed-on: https://go-review.googlesource.com/34274
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

go/loader/example_test.go

@@ -173,7 +173,7 @@ func ExampleConfig_Import() {
 	// created: [strconv_test]
 	// imported: [errors strconv unicode/utf8]
 	// initial: [errors strconv strconv_test unicode/utf8]
-	// all: [bufio bytes context errors flag fmt internal/race io log math math/rand os reflect runtime runtime/debug runtime/internal/atomic runtime/internal/sys runtime/trace sort strconv strconv_test strings sync sync/atomic syscall testing time unicode unicode/utf8 unsafe]
+	// all: [bufio bytes errors flag fmt internal/race io log math math/rand os reflect runtime runtime/debug runtime/internal/atomic runtime/internal/sys runtime/trace sort strconv strconv_test strings sync sync/atomic syscall testing time unicode unicode/utf8 unsafe]
 	// strconv.Files: [atob.go atof.go atoi.go decimal.go doc.go extfloat.go ftoa.go isprint.go itoa.go quote.go internal_test.go]
 	// strconv_test.Files: [atob_test.go atof_test.go atoi_test.go decimal_test.go example_test.go fp_test.go ftoa_test.go itoa_test.go quote_test.go strconv_test.go]
 }