Tmain: add a case verifying ctags provides fields vista.vim expects

<REMOVE ME>
This is an example for a test case verifying ctags satisfies an expection of a client tool.

* See <https://docs.ctags.io/en/latest/tips.html#tmain-a-facility-for-testing-main-part> to
  write and run a test case for Tmain target.
* Use `Tmain: ` as the prefix of the header of the commit log.
* Put files composint a test case to Tmain/client-<the name of client>-<aspect of testing>.d/ .
* Put a README Tmain/client-<the name of client>-<aspect of testing>.d/ in addition to putting
  the standard Tmain test files like run.sh. The README is the message to developers of u-ctags.
* Don't include unessary white-spaces in README and run.sh.
</REMOVE ME>

<REMOVE ME: MY TODO>
I must write this to our docs.
</REMOVE ME: MY TODO>

Tmain/client-vista-vim-fields-expectation.d/README

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+If your change breaks this test case, notify the change
+to https://github.com/liuchengxu/vista.vim.

Tmain/client-vista-vim-fields-expectation.d/exit-expected.txt

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+0

Tmain/client-vista-vim-fields-expectation.d/run.sh

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+# Copyright: 2021 <YOUR NAME HERE:Please put your ame>
+# License: GPL-2
+
+CTAGS=$1
+
+. ../utils.sh
+
+fields=nksSaf
+
+char_expand()
+{
+	if test -n "$1"; then
+		echo ${1:0:1} $(char_expand ${1:1})
+	fi
+}
+
+s=0
+for x in $(char_expand nksSaf); do
+	if ${CTAGS} --quiet --options=NONE --fields=+$x --_fatal-warnings /dev/null; then
+		echo field $x is available.
+	else
+		echo field $x is not available.
+		s=1
+	fi
+done
+
+exit $s

Tmain/client-vista-vim-fields-expectation.d/stdout-expected.txt

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+field n is available.
+field k is available.
+field s is available.
+field S is available.
+field a is available.
+field f is available.