firestore: test cleanups
firestore: test cleanups

- Add underscores next to several test names, so that TestToProtoValue
(now called TestToProtoValue_Conversions) can be run independently.
(basic string matching with `go test -run` causes other tests to be run
currently)
- Rename TestToProtoValue to TestToProtoValue_Conversions to distinguish
it from the other TestToProtoValue tests.
- Update TestToProtoValue_Conversions to use subtests, have meaningful
names (which also document what each test is doing).

Change-Id: I73d38148e54ed98d53a94e7390420091eee5855d
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40995
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go