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, 16 insertions, 0 deletions

diff --git a/pkg/build/testlinux.go b/pkg/build/testlinux.go
new file mode 100644
index 000000000..482c162c5
--- /dev/null
+++ b/pkg/build/testlinux.go
@@ -0,0 +1,16 @@
+// Copyright 2019 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package build
+
+// TypeBuilder implements the builder interface.
+type testBuilder struct{}
+
+func (tb testBuilder) build(targetArch, vmType, kernelDir, outputDir, compiler, userspaceDir,
+	cmdlineFile, sysctlFile string, config []byte) error {
+	return nil
+}
+
+func (tb testBuilder) clean(string, string) error {
+	return nil
+}