Clear symbol table before running unit tests for analyzer

[taichi-ishitani]

Some unit tests for analyzer are not executed correctly because there are cases that an error which has been reported will be reported again.
To avoid this problem, symbol table should be cleared before running a test case.

veryl/crates/analyzer/src/tests.rs

Lines 1680 to 1690 in 801ab8f

	let code = r#"
	module ModuleC {
	always_comb {
	var a: logic;
	a = 1;
	}
	}
	"#;
	let errors = analyze(code);
	assert!(matches!(errors[0], AnalyzerError::UnusedVariable { .. }));

This test case is not executed correctly due to this change so will be removed for now.
#940 is to discuss how to handle this kind of code.