Remove Mandatory from Test-PoshGitImportedInScript -Path

[dahlbyk]

Fix #381

full error trace is available here: https://gist.github.com/choco-bot/8fd09f341ed80aa5cf6ae01f39b49763

2017-01-30 18:46:34,436 [ERROR] - ERROR: Cannot bind argument to parameter 'Path' because it is an empty string.
 at Add-PoshGitToProfile, C:\tools\poshgit\dahlbyk-posh-git-9f6b256\src\Utils.ps1: line 98
at <ScriptBlock>, C:\tools\poshgit\dahlbyk-posh-git-9f6b256\install.ps1: line 6
at <ScriptBlock>, C:\ProgramData\chocolatey\lib\poshgit\tools\chocolateyInstall.ps1: line 42
at <ScriptBlock>, C:\ProgramData\chocolatey\helpers\chocolateyScriptRunner.ps1: line 48
at <ScriptBlock>, <No file>: line 1

As of #376, install.ps1 passes $WhatIf on to Add-PoshGitToProfile, which matches a hack added to let tests override $profilePath. Ultimately this causes $profilePath to be empty when passed as a Mandatory parameter to Test-PoshGitImportedInScript. Two-part fix:

1. Test-PoshGitImportedInScript can correctly handle a $null/empty $Path, so it's safer to remove Mandatory.
2. We can skip the test argument hack by checking the argument type.

[dahlbyk]

@rkeithhill this question is for you. Can we just make $ProfilePath and real (optional) argument that is documented to override $AllHosts?

[dahlbyk]

Alternatively, we could use environment variables to override.

[rkeithhill]

I'd rather not make the public parameters any more complicated for the sake of tests. Yes, we could use env vars but those are global to the process and could impact tests if they ever ran in parallel (which they currently don't).