cmd/go: mark regexp as dependency of testmain

Fixes #6844.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/97840043

src/cmd/go/test.bash

@@ -708,12 +708,28 @@ if ./testgo test notest >/dev/null 2>&1; then
 fi
 unset GOPATH
 
+<<<<<<< local
+TEST 'Issue 6844: cmd/go: go test -a foo does not rebuild regexp'
+if ! ./testgo test -x -a -c testdata/dep_test.go 2>deplist; then
+	echo "go test -x -a -c testdata/dep_test.go failed"
+	ok=false
+elif ! grep -q regexp deplist; then
+	echo "go test -x -a -c testdata/dep_test.go did not rebuild regexp"
+=======
 TEST list template can use context function
 if ! ./testgo list -f "GOARCH: {{context.GOARCH}}"; then 
 	echo unable to use context in list template
+>>>>>>> other
 	ok=false
 fi
+<<<<<<< local
+rm -f deplist
+rm -f deps.test
+=======
+>>>>>>> other
 
+<<<<<<< local
+=======
 TEST build -i installs dependencies
 d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
 export GOPATH=$d
@@ -748,6 +764,7 @@ fi
 rm -rf $d
 unset GOPATH
 
+>>>>>>> other
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/test.go

@@ -418,6 +418,7 @@ func runTest(cmd *Command, args []string) {
 			var coverFiles []string
 			coverFiles = append(coverFiles, p.GoFiles...)
 			coverFiles = append(coverFiles, p.CgoFiles...)
+			coverFiles = append(coverFiles, p.TestGoFiles...)
 			p.coverVars = declareCoverVars(p.ImportPath, coverFiles...)
 		}
 	}
@@ -676,7 +677,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 	stk.push("testmain")
 	for dep := range testMainDeps {
 		if ptest.ImportPath != dep {
-			p1 := loadImport("testing", "", &stk, nil)
+			p1 := loadImport(dep, "", &stk, nil)
 			if p1.Error != nil {
 				return nil, nil, nil, p1.Error
 			}

src/cmd/go/testdata/dep_test.go

@@ -0,0 +1,7 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deps
+
+import _ "testing"