Allow optional strict checking of variable names in EQL expressions

[faec]

This fixes the issue in #2607 where spec file prevention conditions with nonexistent variable names are silently skipped, which led to conditions being unknowingly skipped because of copy-paste errors. It adds a flag allowMissingVariables to the EQL evaluation functions, and sets it to false when evaluating spec file preventions. (I left it true when evaluating upgrade conditions and AST conditions which I believe are used in autodiscovery, to avoid breaking code that depends on the current behavior, though those call sites might deserve a closer look at some point.)

Once invalid variables in spec files became an error, some of the existing tests started failing because of the variable errors from #2606, so I folded that change into this one and added full tests.

Fixes #2607.

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/fragments using the changelog tool
• I have added an integration test or an E2E test

[mergify]

This pull request does not have a backport label. Could you fix it @faec? 🙏
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.

[elasticmachine]

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• Start Time: 2023-05-15T14:22:29.791+0000

• Duration: 20 min 20 sec

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Test	Results
Failed	0
Passed	5659
Skipped	19
Total	5678

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[faec]

an encouraging CI failure :-)

component_test.go:1824: 
        	Error Trace:	/home/runner/work/elastic-agent/elastic-agent/pkg/component/component_test.go:1824
        	Error:      	Not equal: 
        	            	expected: &component.ErrInputRuntimeCheckFail{message:"No support for RHEL7 on arm64"}
        	            	actual  : &component.ErrInputRuntimeCheckFail{message:"unknown variable at line 1 column 2: \"runtime.user.root\""}

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.611% (71/72)	👍
Files	68.273% (170/249)	👍
Classes	67.021% (315/470)	👍
Methods	53.652% (955/1780)	👍 0.052
Lines	39.406% (10958/27808)	👎 -0.028
Conditionals	100.0% (0/0)	💚

[ycombinator]

LGTM. Nice addition!