pkg/bisect: add initial testing support for cause bisection

(note: incomplete change)

Refactor existing code as follows:
* Move reusable test utility functions from git_repo_test.go to
pkg/vcs/test_util.go and make them exported.
* Split Run() into Run()+runImpl().
* Change type of bisect.go:env.inst to `instance.BuilderTester`.
Change usage inside syz-testbuild/testbuild.go accordingly.
* Move most of linux.PreviousReleaseTags() into vcs/git.go as
git.previousReleaseTags().
* Allow build.CompilerIdentity to be mocked.

Introduce the following changes:
* instance.BuilderTester is an interface with methods
	BuildSyzkaller()
	BuildKernel()
	Test()
NewEnv() now returns this interface.
* type testEnv implements instance.BuilderTester.
* type testBuilder implements builder interface. Add a entry into table
inside pkg/build/build.go:getBuilder() to return testBuilder object.

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/syz-testbuild/testbuild.go b/tools/syz-testbuild/testbuild.go
index 81a3466e4..120a465c3 100644
--- a/tools/syz-testbuild/testbuild.go
+++ b/tools/syz-testbuild/testbuild.go
@@ -120,7 +120,7 @@ func main() {
 	}
 }
 
-func test(repo vcs.Repo, bisecter vcs.Bisecter, kernelConfig []byte, env *instance.Env, com *vcs.Commit) {
+func test(repo vcs.Repo, bisecter vcs.Bisecter, kernelConfig []byte, env instance.BuilderTester, com *vcs.Commit) {
 	bisectEnv, err := bisecter.EnvForCommit(com.Hash, kernelConfig)
 	if err != nil {
 		fail(err)