Nimble test suite should change PATH so that Nimble exe is in there for certain tests
I started to work on this [issue](https://github.com/nim-lang/nimble/issues/127) and as a first step, I run the tests to see whether they pass, but there was a problem. One of the tests is failing `[FAILED] issues #280 and #524`  and also after the last test, the tester exits with an error:
```
[Suite] Module tests
  [OK] version
  [OK] reversedeps
  [OK] packageparser
  [OK] packageinfo
  [OK] cli
Error: execution of an external program failed: '/home/bobeff/projects/Nim/dist/nimble/tests/tester '
stack trace: (most recent call last)
/tmp/nimblecache/nimscriptapi.nim(164, 16)
/home/bobeff/projects/Nim/dist/nimble/nimble_83013.nims(30, 10) testTask
/home/bobeff/projects/Nim/lib/system/nimscript.nim(246, 7) exec
/home/bobeff/projects/Nim/lib/system/nimscript.nim(246, 7) Error: unhandled exception: FAILED: nim c -r tester
     Error: Exception raised during nimble script execution
```
This happens with the latest commit in master, as well with the latest tagged commit, both on Windows and Linux. I want to know whether this is expected behavior with the current Nimble? I looked that there is no other tests after the last one reported. Maybe the error at the end is due to the single failing test above, but it is misleading , because I wasn't sure whether there are more tests after it. I checked that the last passed test is really the last defined test, but in the beginning my impression was, that the testing procedure is halted somewhere in the middle. Maybe this should be fixed to report the number of passed and failed tests and to always exit with success?

**Additional information:**
```
$ nim --version
Nim Compiler Version 0.20.99 [Linux: amd64]
Compiled at 2019-07-12
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 2895ad70c8e65cd6c220cf06c5e04ce7adfd29bd
active boot switches: -d:release
```
```
$ nimble --version
nimble v0.10.2 compiled at 2019-07-12 15:13:17
git hash: d15c8530cb7480ce39ffa85a2dd9819d2d4fc645
```

The result is the same on Windows, and with the latest Nimble commit in the `master` branch: ` 5e72840336c03ef66557f96623fedddece53083d `