Fix permissions for config files used in tests

[AndersonQ]

Proposed commit message

fix permissions for config files used in tests

The tests which rely on reading config from disk did not correctly set the 
permissions. The beats require the config files must be writable only by the 
owner, now the tests explicitly set the permissions accordingly.

A new `testcfg` package provides `NewFileWith644Perm` and
`CopyDirectoryWithOwnerWriteOnly` as helper functions for tests to create 
a temporary copy of the files committed to the repo to use in the test.

The test framework was also adjusted to save the config file with the correct 
permission.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

• N/A

How to test this PR locally

tl;dr: no unit/system test should fail due to a configuration file with the wrong permissions.

• set the system umask to 002
• run the tests with the new test framework:

cd filebeat/testing/integration
go test -v -tags integration -run TestFilebeat

• it should succeed, before it'd fail with:

[...]
        Exiting: error loading config file: config file ("/tmp/TestFilebeatFilebeat_starts_and_ingests_filesplain_text_files1677938519/002/filebeat.yml") can only be writable by the owner but the permissions are "-rwxrwxr-x" (to fix the permissions use: 'chmod go-w /tmp/TestFilebeatFilebeat_starts_and_ingests_filesplain_text_files1677938519/002/filebeat.yml')
[...]

• try some of the other unit tests, but first ensure the configuration files have the wrong permissions

cd metricbeat/mb/testdata
chmod -R go+w ./
go test -run TestProcessorsForMetricSet_FromSource ./

• it should succeed. Before it's fail with:

❯ go test -run TestProcessorsForMetricSet_FromSource ./              
--- FAIL: TestProcessorsForMetricSet_FromSource (0.00s)
    registry_test.go:245: 
        	Error Trace:	/home/ainsoph/devel/github.com/elastic/beats/metricbeat/mb/registry_test.go:245
        	Error:      	Received unexpected error:
        	            	reading processors for metricset 'withprocessors' in module 'unpack': loading light module 'unpack' definition: loading module configuration from 'testdata/lightmodules/unpack/module.yml': config file ("testdata/lightmodules/unpack/module.yml") can only be writable by the owner but the permissions are "-rw-rw-r--" (to fix the permissions use: 'chmod go-w /home/ainsoph/devel/github.com/elastic/beats/metricbeat/mb/testdata/lightmodules/unpack/module.yml')
        	Test:       	TestProcessorsForMetricSet_FromSource
FAIL
FAIL	github.com/elastic/beats/v7/metricbeat/mb	0.082s
FAIL

Related issues

• Closes Test framework incorrectly creates beat config file #42779

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @AndersonQ? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)