[Ops] Make unit-test agent agnostic (elastic#174441)

## Summary
This is not an error now but will be required for
elastic/kibana-operations#15 - this solution
should work in both scenarios, so it can be done up-front.

Quote from Brian's notes regarding buildkite migration:
>The buildkite-agent process is running with a different umask in the
new system. Files are being created with slightly different permissions
by default. For example, when the kibana repo is checked out, files have
664 permissions instead of 644. This shouldn’t cause any major issues
for Kibana, BUT a few tests check the permission flags on files during a
copy process. They should probably be changed to check the permissions
on the source file rather than just hard-coding them.

This is showing up in the unit tests like this: 
```
FAIL  src/cli_plugin/install/zip.test.js
--
  | ● kibana cli › zip › checkFilePermission › verify consistency of modes of files
  |  
  | expect(received).toEqual(expected) // deep equality
  |  
  | Expected: "755"
  | Received: "775"
```

We can't access the source files inside the zip file, the rights will
only be revealed after unzipping. However, the difference in the default
access rights is in the RW and not the X, which we're really curious
about.

So as a solution, I'm only checking if the executable file is indeed
executable by all, and the non-executable is not executable by anyone.

src/cli_plugin/install/zip.test.js

@@ -15,7 +15,8 @@ import globby from 'globby';
 
 import { analyzeArchive, extractArchive } from './zip';
 
-const getMode = (path) => (fs.statSync(path).mode & parseInt('777', 8)).toString(8);
+const getExecFlags = (path) =>
+  (fs.statSync(path).mode & parseInt('111', 8)).toString(8).padStart(3, '0');
 
 describe('kibana cli', function () {
   describe('zip', function () {
@@ -83,8 +84,8 @@ describe('kibana cli', function () {
           ]
         `);
 
-        expect(getMode(path.resolve(tempPath, 'executable'))).toEqual('755');
-        expect(getMode(path.resolve(tempPath, 'not-executable'))).toEqual('644');
+        expect(getExecFlags(path.resolve(tempPath, 'executable'))).toEqual('111');
+        expect(getExecFlags(path.resolve(tempPath, 'not-executable'))).toEqual('000');
       });
     });