Use a variable instead of a file for testing args

* Use a variable (which can be indexed by line) instead of a file to
  test the output of arguments passed to echoargs
* Add comments explaining the tests

test/powershell/Scripting/NativeExecution/NativeCommandArguments.Tests.ps1

@@ -1,22 +1,41 @@
 Describe "Native Command Arguments" -tags "CI" {
-    $testPath = 'TestDrive:\test.txt'
+    # Find where test/powershell is so we can find the echoargs command relative to it
     $powershellTestDir = $PSScriptRoot
     while ($powershellTestDir -notmatch 'test[\\/]powershell$') {
         $powershellTestDir = Split-Path $powershellTestDir
     }
     $echoArgs = Join-Path (Split-Path $powershellTestDir) tools/EchoArgs/bin/echoargs
 
+    # When passing arguments to native commands, quoted segments that contain
+    # spaces need to be quoted with '"' characters when they are passed to the
+    # native command (or to bash or sh on Linux).
+    #
+    # This test checks that the proper quoting is occuring by passing arguments
+    # to the echoargs native command and looking at how it got the arguments.
     It "Should handle quoted spaces correctly" {
         $a = 'a"b c"d'
-        & $echoArgs $a 'a"b c"d' a"b c"d >$testPath
+        $lines = & $echoArgs $a 'a"b c"d' a"b c"d
-        $testPath | Should Contain 'Arg 0 is <ab cd>'
+        ($lines | measure).Count | Should Be 3
-        $testPath | Should Contain 'Arg 1 is <ab cd>'
+        $lines[0] | Should Be 'Arg 0 is <ab cd>'
-        $testPath | Should Contain 'Arg 2 is <ab cd>'
+        $lines[1] | Should Be 'Arg 1 is <ab cd>'
+        $lines[2] | Should Be 'Arg 2 is <ab cd>'
     }
 
+    # In order to pass '"' characters so they are actually part of command line
+    # arguments for native commands, they need to be escaped with a '\' (this
+    # is in addition to the '`' escaping needed inside '"' quoted strings in
+    # PowerShell).
+    #
+    # This functionality was broken in PowerShell 5.0 and 5.1, so this test
+    # will fail on those versions unless the fix is backported to them.
+    #
+    # This test checks that the proper quoting and escaping is occurring by
+    # passing arguments with escaped quotes to the echoargs native command and
+    # looking at how it got the arguments.
     It "Should handle spaces between escaped quotes" {
-        & $echoArgs 'a\"b c\"d' "a\`"b c\`"d" >$testPath
+        $lines = & $echoArgs 'a\"b c\"d' "a\`"b c\`"d"
-        $testPath | Should Contain 'Arg 0 is <a"b c"d>'
+        ($lines | measure).Count | Should Be 2
-        $testPath | Should Contain 'Arg 1 is <a"b c"d>'
+        $lines[0] | Should Be 'Arg 0 is <a"b c"d>'
+        $lines[1] | Should Be 'Arg 1 is <a"b c"d>'
     }
 }