cpp backend is not used for "nimble test"
(I'm very close to `devel` in Nim, basically up-to-date.)

`nim cpp -r tests/t_pbbam.nim` works fine, but `nimble test` fails.

```
% nimble test --verbose
...
Hint: g++   -o /localdisk/scratch/cdunn/repo/nim-pbbam/tests/t_pbbam  /home/UNIXHOME/cdunn/.cache/nim/t_pbbam_d/stdlib_assertions.c.o 
... /home/UNIXHOME/cdunn/.cache/nim/t_pbbam_d/stdlib_unittest.c.o /home/UNIXHOME/cdunn/.cache/nim/t_pbbam_d/pbbam_pbbam.cpp.o /home/UNIXHOME/cdunn/.cache/nim/t_pbbam_d/pbbam_t_pbbam.c.o  -lm -lrt /mnt/software/h/htslib/1.9/lib/libhts.so /mnt/software/z/zlib/1.2.11/lib/libz.so -L/mnt/software/p/pbbam/develop/lib -lpbbam     -ldl [Exec]
/home/UNIXHOME/cdunn/.cache/nim/t_pbbam_d/pbbam_t_pbbam.c.o: In function `PreMainInner':
/mnt/software/n/nim/0.19.9/Nim/lib/pure/unittest.nim:185: undefined reference to `pbbam_pbbamInit000'
```
That is defined in a C++ file `.cache/nim/t_pbbam_d/pbbam_pbbam.cpp`, not as `extern "C"`:
```c
N_LIB_PRIVATE N_NIMCALL(void, pbbam_pbbamInit000)(void) {
...
```
The test of course works fine when I comment out my import:
```nim
import unittest
#  import pbbam/pbbam

test "sample":
    check 10 == 10
```
From `pbbam.nimble`, note `backend=cpp`:
```nim
backend       = "cpp"
version       = "0.24.0"
srcDir        = "src"
installDirs   = @["."]
bin           = @["pbdataset"]

requires "nim >= 0.19.6"
```
I think the problem is this:
```
  Compiling /localdisk/scratch/cdunn/repo/nim-pbbam/tests/t_pbbam.nim (from package pbbam) using c backend
  Executing "/mnt/software/n/nim/0.19.9/Nim/bin/nim" c --noNimblePath --path:"/scratch/cdunn/repo/nim-falcon/nimbleDir/pkgs/cligen-0.9.19" "-r" "--path:."  "/localdisk/scratch/cdunn/repo/nim-pbbam/tests/t_pbbam.nim"
```

`nimble install` works fine, since it does use the `cpp` backend.