Catch's ApprovalTests test fail if build directory was reached via a symbolic link

**Describe the bug**
When building/testing Catch2 in a directory that was reached via a symbolic link, the `ApprovalTests xml.sw` test fails.

Here's an example failure line from `build/Testing/Temporary/LastTest.log` which illustrates the gist of the problem:

```
+
-

```

Notice that the directory path has not been correctly sanitized, i.e., it still contains the `/home/dj/tmp/Catch2-2.9.1` prefix.

In this case, `/home/dj/tmp` is a symbolic link to the fully resolved directory name, `/data/macabretmp/dj/`. The `scripts/approvalTests.py` program, which is responsible for this test, uses this code in `scripts/scriptCommon.py` to determine the prefix to strip:
```
catchPath = os.path.dirname(os.path.realpath( os.path.dirname(sys.argv[0])))
```
which in my case is equivalent to setting it to `/data/macabretmp/dj`, not to the required value of `/home/dj/tmp`.

One approach to avoiding this would be for `scripts/scriptCommon.py` to parse the directory from one of the CMake artifacts in the build directory.

**Expected behavior**

I expect that the tests work regardless of whether or not the directory is reached via a symbolic link.

**Platform information:**

- OS: **Debian 9**
- Compiler+version: **GCC v6.3.0**
- Catch version: **v2.9.1**

Start with scripts/approvalTests.py and scripts/scriptCommon.py, especially how catchPath is derived. Reproduce the ApprovalTests xml.sw failure from a build directory reached through a symbolic link, then run the related test and confirm that generated paths are sanitized consistently in both direct and symlinked paths.

Written by the indexing model from the issue text.