Segfault when attempting to `test --shell` a non-test build target

[chrisnovakovic]

MWE:

genrule(
    name = "not_a_test",
    cmd = "true",
)

$ plz test //:not_a_test
0 test targets and 0 tests run; 0 passed.
Total time: 10ms real, 0s compute.

but:

$ plz test //:not_a_test --shell
Temp directories prepared, total time 10ms:
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xc36519]

goroutine 98 [running]:
github.com/thought-machine/please/src/core.(*BuildTarget).GetTestCommand(...)
        src/core/build_target.go:1505
github.com/thought-machine/please/src/output.printTempDirs(0xc0004f0000, 0x7ffe3227e0c6?, 0x1, 0x0)
        src/output/shell_output.go:429 +0x3d9
github.com/thought-machine/please/src/output.MonitorState(0xc0004f0000, 0x0, 0x1, 0x0, 0x1, 0x0, {0x0, 0x0})
        src/output/shell_output.go:89 +0xb87
main.runPlease.func1()
        src/please.go:1214 +0x46
created by main.runPlease in goroutine 1
        src/please.go:1213 +0x4d9

In the latter case we should probably be checking that test_cmd is actually defined.