Fix revive warning in testmain.go files (#3958)

When configuring nogo() to run the revive linter, we currently see
builds of testmain.go files fail with the following error:

    if block ends with call to os.Exit function, so drop this else and outdent its block (move short variable declaration to its own line if necessary)

Let's restructure this code slightly, so that this is no longer an
issue.

go/tools/builders/generate_test_main.go

@@ -167,14 +167,14 @@ func testsInShard() []testing.InternalTest {
 func main() {
 	if bzltestutil.ShouldWrap() {
 		err := bzltestutil.Wrap("{{.Pkgname}}")
+		exitCode := 0
 		if xerr, ok := err.(*exec.ExitError); ok {
-			os.Exit(xerr.ExitCode())
+			exitCode = xerr.ExitCode()
 		} else if err != nil {
 			log.Print(err)
-			os.Exit(bzltestutil.TestWrapperAbnormalExit)
-		} else {
-			os.Exit(0)
+			exitCode = bzltestutil.TestWrapperAbnormalExit
 		}
+		os.Exit(exitCode)
 	}
 
 	testDeps :=