testament spec: `targets` doesn't work well with `cmd`

[timotheecour]

I found this while working on #16343

3 bugs:

• only 1 nim invocation should occur
• PASS: tests/compiler/tcppCompileToNamespace.nim c is misleading because it actually uses cpp backend
• if you use nim $target instead of nim cpp, the test will not honor targets: "cpp" (and fail as expected, for reasons unrelated to this issue) and will attempt c target

Example

from tests/compiler/tcppCompileToNamespace.nim

discard """
cmd: "nim cpp --cppCompileToNamespace:foo $options -r $file"
targets: cpp
"""
import os
echo "a" / "b"
doAssert defined(cpp)

Current Output

PASS: tests/compiler/tcppCompileToNamespace.nim c                  ( 0.18 sec)
PASS: tests/compiler/tcppCompileToNamespace.nim cpp                ( 0.17 sec)

Expected Output

PASS: tests/compiler/tcppCompileToNamespace.nim cpp ( 0.17 sec)

Additional Information

devel 1.5.1 0b73106