test: allow to run tests on the machine without proper go env

[kayrus]

Now all you need are only go and gofmt binaries.

Before test.sh failed with the following msg:

$ ./test.sh 
ls: cannot access /src/github.com/cloudflare/cfssl: No such file or directory

And even with the GOPATH:

$ ./test.sh 
api          cli     Dockerfile          initca     scan       ubiquity
auth         cmd     Dockerfile.build    LICENSE    script     vendor
BUILDING.md  config  Dockerfile.minimal  log        selfsign   whitelist
bundler      crl     errors              multiroot  signer
certdb       crypto  gopath              ocsp       testdata
certinfo     csr     helpers             README.md  test.sh
CHANGELOG    doc     info                revoke     transport
can't load package: package _/home/user/git/cfssl/api: cannot find package "_/home/user/git/cfssl/api" in any of:
        /home/user/go/src/_/home/user/git/cfssl/api (from $GOROOT)
        /home/user/git/cfssl/gopath/src/_/home/user/git/cfssl/api (from $GOPATH)
can't load package: package _/home/user/git/cfssl/api/bundle: cannot find package "_/home/user/git/cfssl/api/bundle" in any of:
        /home/user/go/src/_/home/user/git/cfssl/api/bundle (from $GOROOT)
        /home/user/git/cfssl/gopath/src/_/home/user/git/cfssl/api/bundle (from $GOPATH)

[codecov-io]

Current coverage is 58.32% (diff: 100%)

Merging #646 into master will not change coverage

@@             master       #646   diff @@
==========================================
  Files            74         74          
  Lines          6402       6402          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits           3734       3734          
  Misses         2309       2309          
  Partials        359        359          

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[kisom]

LGTM, thanks!