test: Add git env test util and tests for amend

[jerry-skydio]

This should make it less likely for functionality to break

Fixes: #7

[jerry-skydio]

Reviews in this chain:
└#230 test: Add git env test util and tests for amend
　└#232 Add upload tests
　　└#236 upload: Don't re-add assignees/reviewers that were removed
　　　└#238 upload: Add pr body source argument

[jerry-skydio]

#	head	base	diff	date	summary
0	3be3de55	d80fc364	diff	Apr 17 15:17 PM	2 files changed, 904 insertions(+)
1	442d5dd1	d80fc364	diff	Apr 21 16:46 PM	1 file changed, 18 insertions(+), 63 deletions(-)
2	d404edf8	d80fc364	diff	Apr 21 16:47 PM	1 file changed, 6 insertions(+), 18 deletions(-)
3	52017799	4c8aee30	rebase	Apr 21 17:07 PM	0 files changed
4	9aec3314	4c8aee30	diff	Apr 21 17:11 PM	1 file changed, 2 insertions(+)
5	674e434d	4c8aee30	diff	Apr 22 19:58 PM	1 file changed, 21 insertions(+), 26 deletions(-)
6	a969bc14	f28b9190	diff	Apr 23 12:38 PM	1 file changed, 4 insertions(+), 4 deletions(-)
7	79f0363b	e7e87173	diff	Apr 23 15:01 PM	0 files changed
8	8b92128e	a248a551	diff	Apr 28 12:17 PM	1 file changed, 6 deletions(-)
9	e6a4cc88	a248a551	diff	Apr 28 12:19 PM	1 file changed, 2 insertions(+)

[brian-kubisiak-skydio]

TEST_AUTHOR_NAME?

[jerry-skydio]

git_ctx.author actually doesn't correspond to git config user.name which is maybe weird -- its what's used for the first part of the branch prefix, so can't have spaces (unlike user.name)