fix(cli): fix a bug in CLI argument parsing (#5646)

This fixes a bug related to parsing of Boolean CLI arguments using
the `--argName=value` type syntax.
We intend to support using that syntax to pass a boolean CLI flag (like
`--watch`) but actually at present the way it is 'parsed' will result in
such flags always being set to true! Not good!

This is because we were setting the value of the boolean CLI flag to
`Boolean('false')` if you pass `--argName=false`, which does _not_
result in a value of `false` but instead a value of `true`. This is
changed to instead use a check that the value equals `'true'` to turn
the value into a boolean, and regression tests are added.

This was noticed when investigating #5640, but that bug actually has to
do with how we convert CLI arguments into a Jest configuration.

src/cli/parse-flags.ts

@@ -173,7 +173,7 @@ const parseCLITerm = (flags: ConfigFlags, args: string[]) => {
 
 /**
  * Normalize a 'negative' flag name, just to do a little pre-processing before
- * we pass it to `setCLIArg`.
+ * we pass it to {@link setCLIArg}.
  *
  * @param flagName the flag name to normalize
  * @returns a normalized flag name
@@ -221,12 +221,19 @@ const setCLIArg = (flags: ConfigFlags, rawArg: string, normalizedArg: string, va
 
   // We're setting a boolean!
   if (readOnlyArrayHasStringMember(BOOLEAN_CLI_FLAGS, normalizedArg)) {
-    flags[normalizedArg] =
+    const parsed =
       typeof value === 'string'
-        ? Boolean(value)
+        ? // check if the value is `'true'`
+          value === 'true'
         : // no value was supplied, default to true
           true;
+
+    flags[normalizedArg] = parsed;
     flags.knownArgs.push(rawArg);
+
+    if (typeof value === 'string' && value !== '') {
+      flags.knownArgs.push(value);
+    }
   }
 
   // We're setting a string!

src/cli/test/parse-flags.spec.ts

@@ -89,6 +89,12 @@ describe('parseFlags', () => {
       expect(flags.knownArgs).toEqual([]);
       expect(flags[cliArg]).toBe(undefined);
     });
+
+    it.each([true, false])(`should set the value with --${cliArg}=%p`, (value) => {
+      const flags = parseFlags([`--${cliArg}=${value}`]);
+      expect(flags.knownArgs).toEqual([`--${cliArg}`, String(value)]);
+      expect(flags[cliArg]).toBe(value);
+    });
   });
 
   describe.each(STRING_CLI_FLAGS)('should parse string flag %s', (cliArg) => {