feat: do not allow x:variable to override x:param

[AirQuick]

With @run-as="external", only /x:description/x:param is supposed to take effect in SUT. x:variable is not.
However, x:variable can override x:param and effectively go into SUT. For example:

With this stylesheet

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="3.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

	<xsl:param name="p" />
	<xsl:template name="get-p">
		<xsl:sequence select="$p" />
	</xsl:template>

	<xsl:param name="v" />
	<xsl:template name="get-v">
		<xsl:sequence select="$v" />
	</xsl:template>

</xsl:stylesheet>

and this XSpec

<?xml version="1.0" encoding="UTF-8"?>
<x:description run-as="external" stylesheet="test.xsl"
	xmlns:x="http://www.jenitennison.com/xslt/xspec">

	<x:param name="p">global-p</x:param>
	<x:variable name="v">global-v</x:variable>

	<x:scenario label="test">
		<x:variable name="p">local-p</x:variable>
		<x:variable name="v">local-v</x:variable>

		<x:scenario label="$p">
			<x:call template="get-p" />
			<x:expect label="test" />
		</x:scenario>

		<x:scenario label="$v">
			<x:call template="get-v" />
			<x:expect label="test" />
		</x:scenario>
	</x:scenario>

</x:description>

• get-p returns local-p, not global-p, because x:variable overrides the value of $p.
• get-v returns '', because x:variable by itself does not take effect in SUT.

This behavior could be considered as "by design". (In that sense, #991 is already partially possible. 😀)
But I think it's confusing. So this pull request prohibits x:variable from overriding /x:description/x:param. With this change, the example above is terminated with an error:

C:\xspec\test>..\bin\xspec.bat test.xspec
Creating Test Stylesheet...

ERROR: x:variable (named p) must not override x:param (named p)
...
*** Error compiling the test suite

Upon implementing #991, I'll extend this rule and prohibit x:variable from overriding //x:scenario/x:param as well.

[galtm]

Looks good. I agree that the override behavior would be confusing and it's better to disallow it.