Implementation of automated cross-F#-version testing mentioned in #386. This is adapted from an existing strategy used in the past for validating cross-CLR-version scenarios.

Approach ("downtarget")
 - Build each test case against vPrevious FSharp.Core (4.3.1.0 for now)
 - Run resulting EXE as-is, it will bind to vPrevious

Approach ("redirect")
 - Build each test case against vPrevious FSharp.Core (4.3.1.0 for now)
 - Run resulting exe inside of process that targets vCurrent, with binding redirects

closes #446

commit 423c10d2550bfbdbb6bedd406e56e3d928dd72cb
Author: latkin <latkin@microsoft.com>
Date:   Mon May 18 15:27:53 2015 -0700

    Removing from CI build

commit e29e727a7b64d2215fca7ee61bd6f5805fe82c3b
Merge: 133a57a db6c198
Author: latkin <latkin@microsoft.com>
Date:   Mon May 18 15:26:27 2015 -0700

    Merge branch 'crosstarget-test' of https://github.com/latkin/visualfsharp into latkin-crosstarget-test

    Conflicts:
    	tests/fsharpqa/Source/CodeGen/EmittedIL/Misc/env.lst

commit db6c19851ee0702961afb8f281dfc0c0b37dde2b
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 17:06:34 2015 -0700

    More script fixes

commit 105051fb713b467eeca305c3fc79e0482b767d74
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 16:29:03 2015 -0700

    Fix for RunTests.cmd

commit 65c84533673cca7dfeb0740bdeae78204700c9e8
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 15:39:30 2015 -0700

    Omit some cases from CI build

commit a86ef97767eb9911e8bc60c3c207d923e9196845
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 14:17:07 2015 -0700

    Add to CI build (can be removed)

commit ab100d28afb9e82be8928985c090f8ff30277be9
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 14:16:49 2015 -0700

    Update log paths

commit 4fe04d69ecbb075f8ddfc376cc8388f20dca9d3a
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 14:12:24 2015 -0700

    Comment, use better name for test suites

commit 907c11d9c927b445e605ad9299f9efec64c195d5
Author: latkin <latkin@microsoft.com>
Date:   Tue May 12 13:58:46 2015 -0700

    Using exe impl of ExecAssembly, much faster, allows for easier platform targeting

commit e5041d4261a062852365279fdd64a448966609a6
Author: latkin <latkin@microsoft.com>
Date:   Mon May 11 17:36:40 2015 -0700

    Support for targeting downlevel, then executing redirected back to vCurrent

commit 3577b5a1a1e979d806cb6d46b92373aaa95ca379
Author: latkin <latkin@microsoft.com>
Date:   Mon May 11 16:38:25 2015 -0700

    Detect obviously incompatible test cases in RunAll, instead of requiring annotation

commit 883e1bcc87a1866b4cedfa145a3b88cda950c4c8
Author: latkin <latkin@microsoft.com>
Date:   Mon May 11 16:21:53 2015 -0700

    Marking more tests that can't be run cross-version

commit 3ed9c094406e905863d8e4dea20a8d9561a32e37
Author: latkin <latkin@microsoft.com>
Date:   Fri May 8 17:53:06 2015 -0700

    Start ignoring certain tests that do not support cross-version

commit ae153f3e6d3ff64e63f333ffbbf643a39f61ac74
Author: latkin <latkin@microsoft.com>
Date:   Fri May 8 17:50:05 2015 -0700

    Add support for auto-tagging a test with 'FSI' based on presence of 'FSIMODE' var

commit 63753f33eb928015e572137e0cb81449e24a9ff4
Author: latkin <latkin@microsoft.com>
Date:   Fri May 8 16:10:39 2015 -0700

    Updates to RunTests for cross-targeting

commit 46c786b145f3eb9092e5c00240d70104573b8e19
Author: latkin <latkin@microsoft.com>
Date:   Fri May 8 15:10:53 2015 -0700

    Fixing specification of compiler flags in tests

    SCFLAGS should be used by individual tests to specify required compiler or fsi flags
    ISCFLAGS and IFSIFLAGS should be reserved only for infrastructure use, they apply globally

    ISCFLAGS was populated with '-g --optimize' (emit debug info and enable
    optimizations), but is now clear.  A few tests now need to specify
    these flags explicitly.  This is a better default - tests should not need
    to rely on or know about globally defined flags.

commit 118c1f08c7c8fa055f088547b65a442417ab565a
Author: latkin <latkin@microsoft.com>
Date:   Tue May 5 17:37:08 2015 -0700

    Start working on cross-targeting automation