Add missing error check when installing agent in endpoint-security tests

[cmacknz]

Fixes panics that should have failed with an error. Noticed in this test run hiding the real error.

--- FAIL: TestInstallAndCLIUninstallWithEndpointSecurity (0.37s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xae37bc]

goroutine 84 [running]:
testing.tRunner.func1.2({0xc59640, 0x18d0b20})
	/usr/local/go/src/testing/testing.go:1526 +0x1c8
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1529 +0x364
panic({0xc59640, 0x18d0b20})
	/usr/local/go/src/runtime/panic.go:884 +0x1f4
github.com/elastic/elastic-agent/testing/integration.TestInstallAndCLIUninstallWithEndpointSecurity(0x400039e340)
	/home/ubuntu/agent/testing/integration/endpoint_security_test.go:82 +0x29c
testing.tRunner(0x400039e340, 0xe260e8)
	/usr/local/go/src/testing/testing.go:1576 +0x104
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1629 +0x370

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label. Could you fix it @cmacknz? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[blakerouse]

Looks good.

[elasticmachine]

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• Start Time: 2023-07-25T18:05:00.498+0000

• Duration: 29 min 48 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6099
Skipped	27
Total	6126

💚 Flaky test report

Tests succeeded.

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.718% (77/78)	👍
Files	68.657% (184/268)	👍
Classes	67.606% (336/497)	👍
Methods	54.222% (1053/1942)	👍
Lines	40.328% (12044/29865)	👎 -0.013
Conditionals	100.0% (0/0)	💚

[cmacknz]

The AMD64 error:

    endpoint_security_test.go:83: 
        	Error Trace:	/home/ubuntu/agent/testing/integration/endpoint_security_test.go:248
        	            				/home/ubuntu/agent/testing/integration/endpoint_security_test.go:83
        	Error:      	Received unexpected error:
        	            	http error response with code 404: {StatusCode:404 Error:Not Found Message:Saved object [epm-packages/endpoint] not found}
        	Test:       	TestInstallAndCLIUninstallWithEndpointSecurity

The ARM64 error:

    endpoint_security_test.go:83: 
        	Error Trace:	/home/ubuntu/agent/testing/integration/endpoint_security_test.go:248
        	            				/home/ubuntu/agent/testing/integration/endpoint_security_test.go:83
        	Error:      	Received unexpected error:
        	            	http error response with code 500: {StatusCode:500 Error:Internal Server Error Message:Error installing endpoint 8.9.0: runtime_exception
        	            		Caused by:
        	            			version_conflict_engine_exception: [data_frame_transform_state_and_stats-endpoint.metadata_current-default-8.9.0]: version conflict, document already exists (current version [1])
        	            		Root causes:
        	            			runtime_exception: Failed to persist transform statistics for transform [endpoint.metadata_current-default-8.9.0]}
        	Test:       	TestInstallAndCLIUninstallWithEndpointSecurity

[cmacknz]

buildkite test it

[cmacknz]

Created elastic/kibana#162639 for the test failures observed above.