feat: add junit support to shellcheck output

This requires us to run shellcheck twice, as the first pass doesn't output anything human readable - both because of the redirect and because shellcheck doesn't directly support it - which increases lint runtime but allows us to have both the junit information and the normal human-readable output.

Note that the checkstyle output does not contain timing info.

tests/checkstyle2junit.xslt

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+  <xsl:output encoding="UTF-8" method="xml"></xsl:output>
+
+  <xsl:template match="/">
+    <testsuite>
+      <xsl:attribute name="tests">
+        <xsl:value-of select="count(.//file)" />
+      </xsl:attribute>
+      <xsl:attribute name="failures">
+        <xsl:value-of select="count(.//error)" />
+      </xsl:attribute>
+      <xsl:for-each select="//checkstyle">
+        <xsl:apply-templates />
+      </xsl:for-each>
+    </testsuite>
+  </xsl:template>
+
+  <xsl:template match="file">
+    <testcase>
+      <xsl:attribute name="classname">
+        <xsl:value-of select="@name" />
+      </xsl:attribute>
+      <xsl:attribute name="name">
+        <xsl:value-of select="@name" />
+      </xsl:attribute>
+      <xsl:apply-templates select="node()" />
+    </testcase>
+  </xsl:template>
+
+  <xsl:template match="error">
+    <failure>
+      <xsl:attribute name="type">
+        <xsl:value-of select="@source" />
+      </xsl:attribute>
+      <xsl:text>Line </xsl:text>
+      <xsl:value-of select="@line" />
+      <xsl:text>: </xsl:text>
+      <xsl:value-of select="@message" />
+      <xsl:text> See https://www.shellcheck.net/wiki/</xsl:text>
+      <xsl:value-of select="substring(@source, '12')" />
+    </failure>
+  </xsl:template>
+</xsl:stylesheet>