strip .exe suffix via case-insensitive replacement (fixes #55 and #56)

[mykmelez]

Strip a '.exe' extension via a case-insensitive replacement rather than via the case-sensitive 'ext` parameter to path.basename because it might be capitalized ('.EXE'). This fixes #56 for me when testing https://github.com/mozilla/qbrt, which uses https://github.com/tapjs/node-tap, which uses https://github.com/istanbuljs/nyc, which uses this package.

[mykmelez]

I'm not sure why three of the four Linux test runs failed. Each run seems to have failed a single test, but the test that failed is different in each case. Perhaps they're unrelated intermittent failures? I can't reproduce them locally, neither on Node 4.8.3 nor Node 6.11.0 (on my Ubuntu 16.04 system). I also can't reproduce on macOS.

The Windows failures are presumably the unrelated ones from #55.

[popomore]

I'm also faced the problem when I run coverage with nyc on Windows, see link below

https://ci.appveyor.com/project/eggjs/egg-mock/build/1.0.377/job/cp0k4bjh54qd6l2t

[popomore]

ping @isaacs

cc @bcoe

[popomore]

@isaacs can you see this PR?

[mykmelez]

@isaacs dbf243a and bfd4bae fix the (unrelated) test failures on Windows that were reported in #55, so this now fixes both #55 and #56.

The current failures are:

1. On Appveyor, the Node 0.12 test run fails on npm install because that version of Node doesn't support fat arrow functions:

npm install
npm WARN npm npm does not support Node.js v0.12.18
npm WARN npm You should probably upgrade to a newer version of node as we
npm WARN npm can't make any promises that npm will work with this version.
npm WARN npm You can find the latest version at https://nodejs.org/
npm ERR! C:\Users\appveyor\AppData\Roaming\npm\node_modules\npm\lib\install.js:298
npm ERR!         [this, (next) => { computeMetadata(this.idealTree); next() }],
npm ERR!                       ^^
npm ERR! Unexpected token =>

2. On Travis, the Node 0.12 test run fails one of the two tests in test/exec-flag.js, which looks like an intermittent failure, since the only changes I made to that file's logic are Windows-specific, and I can't reproduce locally on my Linux box.

[mykmelez]

It looks like @isaacs decided to fix the issues himself instead of taking this pull request, so I'm closing it.