PS: Add test cases.

Author: MathiasVP

powershell/ql/test/library-tests/dataflow/global/test.expected

@@ -0,0 +1,6 @@
+models
+edges
+nodes
+subpaths
+testFailures
+#select

powershell/ql/test/library-tests/dataflow/global/test.ps1

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+function WriteToEnvVar {
+    $env:x = Source "1"
+}
+
+function ReadFromEndVar {
+    Sink $Env:x # $ MISSING: hasValueFlow=1 hasValueFlow=3
+}
+
+function WriteAndThenOverWriteEnvVar {
+    $env:x = Source "2"
+    $env:x = 0
+}
+
+function MaybeWriteToEnvVar($b) {
+    if($b -eq $true) {
+        $env:x = Source "3"
+    } else {
+        $env:x = 0
+    }
+}

powershell/ql/test/library-tests/dataflow/global/test.ql

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+/**
+ * @kind path-problem
+ */
+
+import powershell
+import semmle.code.powershell.dataflow.DataFlow
+private import TestUtilities.InlineFlowTest
+import DefaultFlowTest
+import ValueFlow::PathGraph
+
+from ValueFlow::PathNode source, ValueFlow::PathNode sink
+where ValueFlow::flowPath(source, sink)
+select sink, source, sink, "$@", source, source.toString()