:tests syntax refuses to run multiple tests

#### Steps to reproduce:

Create a multi-package project, each package containing a test suite with name **tests**.

Then run:

```
$ stack build :tests
Error parsing targets: Ambiugous component name tests, matches: [(package-a,CTest "tests"),(package-b,CTest "tests")]
```
#### Expected:

The tests suites of both packages would be built and executed.
#### Actual:

stack refuses to build any tests. Instead one has to list all the packages explicitly:

```
stack build package-a:tests package-b:tests
```