t3400 (rebase): don't set GIT_AUTHOR_{NAME,EMAIL}

test-lib.sh already sets a sane GIT_AUTHOR_{NAME,EMAIL} for all test
scripts to use.  Don't unnecessarily duplicate the work.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t3400 (rebase): downcase a couple of test titles

Otherwise they stick out like sore thumbs in the test output, where
all the other titles begin with a lowercase letter.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t3400 (rebase): move lone statement into a test

'rm -f B' is a lone statement that isn't contained inside any
test_expect_* block.  Use test_when_finished to execute it after the
preceding test has finished.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t4300 (rebase): don't unnecessarily set GIT_TRACE

A couple of tests execute 'git rebase' with GIT_TRACE set to 1, but
this trace output is not used anywhere.  Remove it, since it is not
relevant to what we are testing.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>