cmd/go: add -coverpkg

The new -coverpkg flag allows computing coverage in
one set of packages while running the tests of a different set.

Also clean up some of the previous CL's recompileForTest,
using packageList to avoid the clumsy recursion.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10705043

src/cmd/go/doc.go

@@ -755,7 +755,12 @@ control the execution of any test:
 		atomic: int: count, but correct in multithreaded tests;
 			significantly more expensive.
 	    Implies -cover.
-	    Sets -v. TODO: This will change.
+
+	-coverpkg pkg1,pkg2,pkg3
+	    Apply coverage analysis in each test to the given list of packages.
+	    If this option is not present, each test applies coverage analysis to
+	    the package being tested. Packages are specified as import paths.
+	    Implies -cover.
 
 	-coverprofile cover.out
 	    Write a coverage profile to the specified file after all tests

src/cmd/go/test.bash

@@ -324,6 +324,11 @@ rm -rf $d
 # Only succeeds if source order is preserved.
 ./testgo test testdata/example[12]_test.go
 
+# Check that coverage analysis works at all.
+# Don't worry about the exact numbers
+./testgo test -coverpkg=strings strings regexp
+./testgo test -cover strings math regexp
+
 # clean up
 rm -rf testdata/bin testdata/bin1
 rm -f testgo

src/cmd/go/testflag.go

@@ -29,6 +29,7 @@ var usageMessage = `Usage of go test:
   -benchtime=1s: passes -test.benchtime to test
   -cover=false: enable coverage analysis
   -covermode="set": passes -test.covermode to test if -cover
+  -coverpkg="": comma-separated list of packages for coverage analysis
   -coverprofile="": passes -test.coverprofile to test if -cover
   -cpu="": passes -test.cpu to test
   -cpuprofile="": passes -test.cpuprofile to test
@@ -67,6 +68,7 @@ var testFlagDefn = []*testFlagSpec{
 	{name: "file", multiOK: true},
 	{name: "i", boolVar: &testI},
 	{name: "cover", boolVar: &testCover},
+	{name: "coverpkg"},
 
 	// build flags.
 	{name: "a", boolVar: &buildA},
@@ -178,6 +180,13 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 			testTimeout = value
 		case "blockprofile", "cpuprofile", "memprofile":
 			testProfile = true
+		case "coverpkg":
+			testCover = true
+			if value == "" {
+				testCoverPaths = nil
+			} else {
+				testCoverPaths = strings.Split(value, ",")
+			}
 		case "coverprofile":
 			testCover = true
 			testProfile = true